Saving/restoring selected registers in transactional processing

ABSTRACT

A TRANSACTION BEGIN instruction begins execution of a transaction and includes a general register save mask having bits, that when set, indicate registers to be saved in the event the transaction is aborted. At the beginning of the transaction, contents of the registers are saved in memory not accessible to the program, and if the transaction is aborted, the saved contents are copied to the registers.

BACKGROUND

One or more aspects relate, in general, to multiprocessing computingenvironments, and in particular, to transactional processing within suchcomputing environments.

An enduring challenge in multiprocessor programming is that of updatesto the same storage location by multiple central processing units(CPUs). Many instructions that update storage locations, including evensimple logical operations, such as AND, do so with multiple accesses tothe location. For instance, first, the storage location is fetched, andthen, the updated result is stored back.

In order for multiple CPUs to safely update the same storage location,access to the location is serialized. One instruction, the TEST AND SETinstruction, introduced with the S/360 architecture formerly offered byInternational Business Machines Corporation, provided an interlockedupdate of a storage location. Interlocked update means that, as observedby other CPUs and the input/output (I/O) subsystem (e.g., channelsubsystem), the entire storage access of the instruction appears tooccur atomically. Later, the S/370 architecture offered by InternationalBusiness Machines Corporation introduced the COMPARE AND SWAP andCOMPARE DOUBLE AND SWAP instructions that provide a more sophisticatedmeans of performing interlocked update, and allow the implementation ofwhat is commonly known as a lock word (or semaphore). Recently addedinstructions have provided additional interlocked-update capabilities,including COMPARE AND SWAP AND PURGE, and COMPARE AND SWAP AND STORE.However, all of these instructions provide interlocking for only asingle storage location.

More complex program techniques may require the interlocked update ofmultiple storage locations, such as when adding an element to adoubly-linked list. In such an operation, both a forward and backwardpointer are to appear to be simultaneously updated, as observed by otherCPUs and the I/O subsystem. In order to effect such a multiple locationupdate, the program is forced to use a separate, single point ofserialization, such as a lock word. However, lock words may provide amuch courser level of serialization than is warranted; for example, thelock words may serialize an entire queue of millions of elements, eventhough only two elements are being updated. The program may structurethe data to use finer-grained serialization (e.g., a hierarchy of lockpoints), but that introduces additional problems, such as potentialdeadlock situations if the hierarchy is violated, and recovery issues ifthe program encounters an error while holding one or more locks or ifthe lock cannot be acquired.

In addition to the above, there are numerous scenarios where a programmay execute a sequence of instructions that may or may not result in anexception condition. If no exception condition occurs, then the programcontinues; however, if an exception is recognized, then the program maytake corrective action to eliminate the exception condition. Java, asone example, can exploit such execution in, for instance, speculativeexecution, partial in-lining of a function, and/or in the re-sequencingof pointer null checking.

In classic operating system environments, such as z/OS and itspredecessors offered by International Business Machines Corporation, theprogram establishes a recovery environment to intercept anyprogram-exception condition that it may encounter. If the program doesnot intercept the exception, the operating system typically abnormallyterminates the program for exceptions that the operating system is notprepared to handle. Establishing and exploiting such an environment iscostly and complicated.

BRIEF SUMMARY

Shortcomings of the prior art are overcome and advantages are providedthrough the provision of a computer program product for facilitatingprocessing of transactions within a computing environment. The computerprogram product includes a computer readable storage medium readable bya processing circuit and storing instructions for execution by theprocessing circuit for performing a method. The method includes, forinstance, determining, by a processor, from information provided by aninstruction one or more selected registers to be saved in transactionalprocessing, the instruction to begin execution of a transaction, thetransaction effectively delaying committing transactional stores to mainmemory until completion of a selected transaction, and the one or moreselected registers being less than all the registers used by thetransaction in execution of the transaction; and saving contents of theone or more selected registers, based on execution of the instruction.

Methods and systems relating to one or more embodiments are alsodescribed and claimed herein. Further, services relating to one or moreembodiments are also described and may be claimed herein.

Additional features and advantages are realized. Other embodiments andaspects are described in detail herein and are considered a part of theclaimed invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects are particularly pointed out and distinctly claimedas examples in the claims at the conclusion of the specification. Theforegoing and other objects, features, and advantages are apparent fromthe following detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 depicts one embodiment of a computing environment;

FIG. 2A depicts one example of a Transaction Begin (TBEGIN) instruction;

FIG. 2B depicts one embodiment of further details of a field of theTBEGIN instruction of FIG. 2A;

FIG. 3A depicts on example of a Transaction Begin constrained (TBEGINC)instruction;

FIG. 3B depicts one embodiment of further details of a field of theTBEGINC instruction of FIG. 3A;

FIG. 4 depicts one example of a Transaction End (TEND) instruction;

FIG. 5 depicts one example of a Transaction Abort (TABORT) instruction;

FIG. 6 depicts one example of nested transactions;

FIG. 7 depicts one example of a NONTRANSACTIONAL STORE (NTSTG)instruction;

FIG. 8 depicts one example of an EXTRACT TRANSACTION NESTING DEPTH(ETND) instruction;

FIG. 9 depicts one example of a transaction diagnostic block;

FIG. 10 depicts example reasons for abort, along with associated abortcodes and condition codes;

FIG. 11 depicts one embodiment of the logic associated with executing aTBEGINC instruction;

FIG. 12 depicts one embodiment of the logic associated with executing aTBEGIN instruction;

FIGS. 13A-13B depict embodiments of the logic associated withsaving/restoring selected registers in transactional processing;

FIGS. 14A-14B depict an example of inserting a queue element into adoubly linked list of queue elements;

FIG. 15 depicts one embodiment of a computer program product;

FIG. 16 depicts one embodiment of a host computer system;

FIG. 17 depicts a further example of a computer system;

FIG. 18 depicts another example of a computer system comprising acomputer network;

FIG. 19 depicts one embodiment of various elements of a computer system;

FIG. 20A depicts one embodiment of the execution unit of the computersystem of FIG. 19;

FIG. 20B depicts one embodiment of the branch unit of the computersystem of FIG. 19;

FIG. 20C depicts one embodiment of the load/store unit of the computersystem of FIG. 19; and

FIG. 21 depicts one embodiment of an emulated host computer system.

DETAILED DESCRIPTION

In accordance with one aspect, a transactional execution (TX) facilityis provided. This facility provides transactional processing forinstructions, and in one or more embodiments, offers different executionmodes, as described below, as well as nested levels of transactionalprocessing.

The transactional execution facility introduces a CPU state called thetransactional execution (TX) mode. Following a CPU reset, the CPU is notin the TX mode. The CPU enters the TX mode by a TRANSACTION BEGINinstruction. The CPU leaves the TX mode by either (a) an outermostTRANSACTION END instruction (more details on inner and outer to follow),or (b) the transaction being aborted. While in the TX mode, storageaccesses by the CPU appear to be block-concurrent as observed by otherCPUs and the I/O subsystem. The storage accesses are either (a)committed to storage when the outermost transaction ends withoutaborting (i.e., e.g., updates made in a cache or buffer local to the CPUare propagated and stored in real memory and visible to other CPUs), or(b) discarded if the transaction is aborted.

Transactions may be nested. That is, while the CPU is in the TX mode, itmay execute another TRANSACTION BEGIN instruction. The instruction thatcauses the CPU to enter the TX mode is called the outermost TRANSACTIONBEGIN; similarly, the program is said to be in the outermosttransaction. Subsequent executions of TRANSACTION BEGIN are called innerinstructions; and the program is executing an inner transaction. Themodel provides a minimum nesting depth and a model-dependent maximumnesting depth. An EXTRACT TRANSACTION NESTING DEPTH instruction returnsthe current nesting depth value, and in a further embodiment, may returna maximum nesting-depth value. This technique uses a model called“flattened nesting” in which an aborting condition at any nesting depthcauses all levels of the transaction to be aborted, and control isreturned to the instruction following the outermost TRANSACTION BEGIN.

During processing of a transaction, a transactional access made by oneCPU is said to conflict with either (a) a transactional access ornontransactional access made by another CPU, or (b) a nontransactionalaccess made by the I/O subsystem, if both accesses are to any locationwithin the same cache line, and one or both of the accesses is a store.In other words, in order for transactional execution to be productive,the CPU is not to be observed making transactional accesses until itcommits. This programming model may be highly effective in certainenvironments; for example, the updating of two points in a doubly-linkedlist of a million elements. However, it may be less effective, if thereis a lot of contention for the storage locations that are beingtransactionally accessed.

In one model of transactional execution (referred to herein as anonconstrained transaction), when a transaction is aborted, the programmay either attempt to re-drive the transaction in the hopes that theaborting condition is no longer present, or the program may “fall back”to an equivalent non-transactional path. In another model oftransactional execution (referred to herein as a constrainedtransaction), an aborted transaction is automatically re-driven by theCPU; in the absence of constraint violations, the constrainedtransaction is assured of eventual completion.

When initiating a transaction, the program can specify various controls,such as (a) which general registers are restored to their originalcontents if the transaction is aborted, (b) whether the transaction isallowed to modify the floating-point-register context, including, forinstance, floating point registers and the floating point controlregister, (c) whether the transaction is allowed to modify accessregisters (ARs), and (d) whether certain program-exception conditionsare to be blocked from causing an interruption. If a nonconstrainedtransaction is aborted, various diagnostic information may be provided.For instance, the outermost TBEGIN instruction that initiates anonconstrained transaction may designate a program specified transactiondiagnostic block (TDB). Further, the TDB in the CPU's prefix area ordesignated by the host's state description may also be used if thetransaction is aborted due to a program interruption or a condition thatcauses interpretative execution to end, respectively.

Indicated above are various types of registers. These are furtherexplained in detail herein. General registers may be used asaccumulators in general arithmetic and logical operations. In oneembodiment, each register contains 64 bit positions, and there are 16general registers. The general registers are identified by the numbers0-15, and are designated by a four-bit R field in an instruction. Someinstructions provide for addressing multiple general registers by havingseveral R fields. For some instructions, the use of a specific generalregister is implied rather than explicitly designated by an R field ofthe instruction.

In addition to their use as accumulators in general arithmetic andlogical operations, 15 of the 16 general registers are also used as baseaddress and index registers in address generation. In these cases, theregisters are designated by a four-bit B field or X field in aninstruction. A value of zero in the B or X field specifies that no baseor index is to be applied, and thus, general register 0 is not to bedesignated as containing a base address or index.

Floating point instructions use a set of floating point registers. TheCPU has 16 floating point registers, in one embodiment. The floatingpoint registers are identified by the numbers 0-15, and are designatedby a four bit R field in floating point instructions. Each floatingpoint register is 64 bits long and can contain either a short (32-bit)or a long (64-bit) floating point operand.

A floating point control (FPC) register is a 32-bit register thatcontains mask bits, flag bits, a data exception code, and rounding modebits, and is used during processing of floating point operations.

Further, in one embodiment, the CPU has 16 control registers, eachhaving 64 bit positions. The bit positions in the registers are assignedto particular facilities in the system, such as Program Event Recording(PER) (discussed below), and are used either to specify that anoperation can take place or to furnish special information required bythe facility. In one embodiment, for the transactional facility, CR0(bits 8 and 9) and CR2 (bits 61-63) are used, as described below.

The CPU has, for instance, 16 access registers numbered 0-15. An accessregister consists of 32 bit positions containing an indirectspecification of an address space control element (ASCE). An addressspace control element is a parameter used by the dynamic addresstranslation (DAT) mechanism to translate references to a correspondingaddress space. When the CPU is in a mode called the access register mode(controlled by bits in the program status word (PSW)), an instruction Bfield, used to specify a logical address for a storage operandreference, designates an access register, and the address space controlelement specified by the access register is used by DAT for thereference being made. For some instructions, an R field is used insteadof a B field. Instructions are provided for loading and storing thecontents of the access registers and for moving the contents of oneaccess register to another.

Each of access registers 1-15 can designate any address space. Accessregister 0 designates the primary instruction space. When one of accessregisters 1-15 is used to designate an address space, the CPU determineswhich address space is designated by translating the contents of theaccess register. When access register 0 is used to designate an addressspace, the CPU treats the access register as designating the primaryinstruction space, and it does not examine the actual contents of theaccess register. Therefore, the 16 access registers can designate, atany one time, the primary instruction space and a maximum of 15 otherspaces.

In one embodiment, there are multiple types of address spaces. Anaddress space is a consecutive sequence of integer numbers (virtualaddresses), together with the specific transformation parameters whichallow each number to be associated with a byte location in storage. Thesequence starts at zero and proceeds left to right.

In, for instance, the z/Architecture, when a virtual address is used bya CPU to access main storage (a.k.a., main memory), it is firstconverted, by means of dynamic address translation (DAT), to a realaddress, and then, by means of prefixing, to an absolute address. DATmay use from one to five levels of tables (page, segment, region third,region second, and region first) as transformation parameters. Thedesignation (origin and length) of the highest-level table for aspecific address space is called an address space control element, andit is found for use by DAT in a control register or as specified by anaccess register. Alternatively, the address space control element for anaddress space may be a real space designation, which indicates that DATis to translate the virtual address simply by treating it as a realaddress and without using any tables.

DAT uses, at different times, the address space control elements indifferent control registers or specified by the access registers. Thechoice is determined by the translation mode specified in the currentPSW. Four translation modes are available: primary space mode, secondaryspace mode, access register mode and home space mode. Different addressspaces are addressable depending on the translation mode.

At any instant when the CPU is in the primary space mode or secondaryspace mode, the CPU can translate virtual addresses belonging to twoaddress spaces—the primary address space and the second address space.At any instant when the CPU is in the access register mode, it cantranslate virtual addresses of up to 16 address spaces—the primaryaddress space and up to 15 AR-specified address spaces. At any instantwhen the CPU is in the home space mode, it can translate virtualaddresses of the home address space.

The primary address space is identified as such because it consists ofprimary virtual addresses, which are translated by means of the primaryaddress space control element (ASCE). Similarly, the secondary addressspace consists of secondary virtual addresses translated by means of thesecondary ASCE; the AR specified address spaces consist of AR specifiedvirtual addresses translated by means of AR specified ASCEs; and thehome address space consists of home virtual addresses translated bymeans of the home ASCE. The primary and secondary ASCEs are in controlregisters 1 and 7, respectively. AR specified ASCEs are inASN-second-table entries that are located through a process calledaccess-register translation (ART) using control registers 2, 5 and 8.The home ASCE is in control register 13.

One embodiment of a computing environment to incorporate and use one ormore aspects of the transactional facility described herein is describedwith reference to FIG. 1.

Referring to FIG. 1, in one example, computing environment 100 is basedon the z/Architecture, offered by International Business Machines (IBM®)Corporation, Armonk, N.Y. The z/Architecture is described in an IBMPublication entitled “z/Architecture—Principles of Operation,”Publication No. SA22-7932-08, 9^(th) Edition, August 2010, which ishereby incorporated herein by reference in its entirety.

Z/ARCHITECTURE, IBM, and Z/OS and Z/VM (referenced below) are registeredtrademarks of International Business Machines Corporation, Armonk, N.Y.Other names used herein may be registered trademarks, trademarks orproduct names of International Business Machines Corporation or othercompanies.

As one example, computing environment 100 includes a central processorcomplex (CPC) 102 coupled to one or more input/output (I/O) devices 106via one or more control units 108. Central processor complex 102includes, for instance, one or more central processors 110, one or morepartitions 112 (e.g., logical partitions (LP)), a logical partitionhypervisor 114, and an input/output subsystem 115, each of which isdescribed below.

Central processors 110 are physical processor resources allocated to thelogical partitions. In particular, each logical partition 112 has one ormore logical processors, each of which represents all or a share of aphysical processor 110 allocated to the partition. The logicalprocessors of a particular partition 112 may be either dedicated to thepartition, so that the underlying processor resource 110 is reserved forthat partition; or shared with another partition, so that the underlyingprocessor resource is potentially available to another partition.

A logical partition functions as a separate system and has one or moreapplications, and optionally, a resident operating system therein, whichmay differ for each logical partition. In one embodiment, the operatingsystem is the z/OS operating system, the z/VM operating system, thez/Linux operating system, or the TPF operating system, offered byInternational Business Machines Corporation, Armonk, N.Y. Logicalpartitions 112 are managed by a logical partition hypervisor 114, whichis implemented by firmware running on processors 110. As used herein,firmware includes, e.g., the microcode and/or millicode of theprocessor. It includes, for instance, the hardware-level instructionsand/or data structures used in implementation of higher level machinecode. In one embodiment, it includes, for instance, proprietary codethat is typically delivered as microcode that includes trusted softwareor microcode specific to the underlying hardware and controls operatingsystem access to the system hardware.

The logical partitions and logical partition hypervisor each compriseone or more programs residing in respective partitions of centralstorage associated with the central processors. One example of logicalpartition hypervisor 114 is the Processor Resource/System Manager(PR/SM), offered by International Business Machines Corporation, Armonk,N.Y.

Input/output subsystem 115 directs the flow of information betweeninput/output devices 106 and main storage (a.k.a., main memory). It iscoupled to the central processing complex, in that it can be a part ofthe central processing complex or separate therefrom. The I/O subsystemrelieves the central processors of the task of communicating directlywith the input/output devices and permits data processing to proceedconcurrently with input/output processing. To provide communications,the I/O subsystem employs I/O communications adapters. There are varioustypes of communications adapters including, for instance, channels, I/Oadapters, PCI cards, Ethernet cards, Small Computer Storage Interface(SCSI) cards, etc. In the particular example described herein, the I/Ocommunications adapters are channels, and therefore, the I/O subsystemis referred to herein as a channel subsystem. However, this is only oneexample. Other types of I/O subsystems can be used.

The I/O subsystem uses one or more input/output paths as communicationlinks in managing the flow of information to or from input/outputdevices 106. In this particular example, these paths are called channelpaths, since the communication adapters are channels.

The computing environment described above is only one example of acomputing environment that can be used. Other environments, includingbut not limited to, non-partitioned environments, other partitionedenvironments, and/or emulated environments, may be used; embodiments arenot limited to any one environment.

In accordance with one or more aspects, the transactional executionfacility is a CPU enhancement that provides the means by which the CPUcan execute a sequence of instructions—known as a transaction—that mayaccess multiple storage locations, including the updating of thoselocations. As observed by other CPUs and the I/O subsystem, thetransaction is either (a) completed in its entirety as a single atomicoperation, or (b) aborted, potentially leaving no evidence that it everexecuted (except for certain conditions described herein). Thus, asuccessfully completed transaction can update numerous storage locationswithout any special locking that is needed in the classicmultiprocessing model.

The transactional execution facility includes, for instance, one or morecontrols; one or more instructions; transactional processing, includingconstrained and nonconstrained execution; and abort processing, each ofwhich is further described below.

In one embodiment, three special purpose controls, including atransaction abort Program Status Word (PSW), a transaction diagnosticblock (TDB) address, and a transaction nesting depth; five controlregister bits; and six general instructions, including TRANSACTION BEGIN(constrained and nonconstrained), TRANSACTION END, EXTRACT TRANSACTIONNESTING DEPTH, TRANSACTION ABORT, and NONTRANSACTIONAL STORE, are usedto control the transactional execution facility. When the facility isinstalled, it is installed, for instance, in all CPUs in theconfiguration. A facility indication, bit 73 in one implementation, whenone, indicates that the transactional execution facility is installed.

When the transactional execution facility is installed, theconfiguration provides a nonconstrained transactional executionfacility, and optionally, a constrained transactional executionfacility, each of which is described below. When facility indications 50and 73, as examples, are both one, the constrained transactionalexecution facility is installed. Both facility indications are stored inmemory at specified locations.

As used herein, the instruction name TRANSACTION BEGIN refers to theinstructions having the mnemonics TBEGIN (Transaction Begin for anonconstrained transaction) and TBEGINC (Transaction Begin for aconstrained transaction). Discussions pertaining to a specificinstruction are indicated by the instruction name followed by themnemonic in parentheses or brackets, or simply by the mnemonic.

One embodiment of a format of a TRANSACTION BEGIN (TBEGIN) instructionis depicted in FIGS. 2A-2B. As one example, a TBEGIN instruction 200includes an opcode field 202 that includes an opcode specifying atransaction begin nonconstrained operation; a base field (B₁) 204; adisplacement field (D₁) 206; and an immediate field (I₂) 208. When theB₁ field is nonzero, the contents of the general register specified byB₁ 204 are added to D₁ 206 to obtain the first operand address.

When the B₁ field is nonzero, the following applies:

-   -   When the transaction nesting depth is initially zero, the first        operand address designates the location of the 256 byte        transaction diagnostic block, called the TBEGIN-specified TDB        (described further below) into which various diagnostic        information may be stored if the transaction is aborted. When        the CPU is in the primary space mode or access register mode,        the first operand address designates a location in the primary        address space. When the CPU is in the secondary space or home        space mode, the first operand address designates a location in        the secondary or home address space, respectively. When DAT is        off, the transaction diagnostic block (TDB) address (TDBA)        designates a location in real storage.    -   Store accessibility to the first operand is determined. If        accessible, the logical address of the operand is placed into        the transaction diagnostic block address (TDBA), and the TDBA is        valid.    -   When the CPU is already in the nonconstrained transactional        execution mode, the TDBA is not modified, and it is        unpredictable whether the first operand is tested for        accessibility.

When the B₁ field is zero, no access exceptions are detected for thefirst operand and, for the outermost TBEGIN instruction, the TDBA isinvalid.

The bits of the I₂ field are defined as follows, in one example:

General Register Save Mask (GRSM) 210 (FIG. 2B):

Bits 0-7 of the I₂ field contain the general register save mask (GRSM).Each bit of the GRSM represents an even-odd pair of general registers,where bit 0 represents registers 0 and 1, bit 1 represents registers 2and 3, and so forth. When a bit in the GRSM of the outermost TBEGINinstruction is zero, the corresponding register pair is not saved. Whena bit in the GRSM of the outermost TBEGIN instruction is one, thecorresponding register pair is saved in a model dependent location thatis not directly accessible by the program.

If the transaction aborts, saved register pairs are restored to theircontents when the outermost TBEGIN instruction was executed. Thecontents of all other (unsaved) general registers are not restored whena transaction aborts.

The general register save mask is ignored on all TBEGINs except for theoutermost one.

Allow AR Modification (A) 212:

The A control, bit 12 of the I₂ field, controls whether the transactionis allowed to modify an access register. The effective allow ARmodification control is the logical AND of the A control in the TBEGINinstruction for the current nesting level and for all outer levels.

If the effective A control is zero, the transaction will be aborted withabort code 11 (restricted instruction) if an attempt is made to modifyany access register. If the effective A control is one, the transactionwill not be aborted if an access register is modified (absent of anyother abort condition).

Allow Floating Point Operation (F) 214:

The F control, bit 13 of the I₂ field, controls whether the transactionis allowed to execute specified floating point instructions. Theeffective allow floating point operation control is the logical AND ofthe F control in the TBEGIN instruction for the current nesting leveland for all outer levels.

If the effective F control is zero, then (a) the transaction will beaborted with abort code 11 (restricted instruction) if an attempt ismade to execute a floating point instruction, and (b) the data exceptioncode (DXC) in byte 2 of the floating point control register (FPCR) willnot be set by any data exception program exception condition. If theeffective F control is one, then (a) the transaction will not be abortedif an attempt is made to execute a floating point instruction (absentany other abort condition), and (b) the DXC in the FPCR may be set by adata exception program exception condition.

Program Interruption Filtering Control (PIFC) 216:

Bits 14-15 of the I₂ field are the program interruption filteringcontrol (PIFC). The PIFC controls whether certain classes of programexception conditions (e.g., addressing exception, data exception,operation exception, protection exception, etc.) that occur while theCPU is in the transactional execution mode result in an interruption.

The effective PIFC is the highest value of the PIFC in the TBEGINinstruction for the current nesting level and for all outer levels. Whenthe effective PIFC is zero, all program exception conditions result inan interruption. When the effective PIFC is one, program exceptionconditions having a transactional execution class of 1 and 2 result inan interruption. (Each program exception condition is assigned at leastone transactional execution class, depending on the severity of theexception. Severity is based on the likelihood of recovery during arepeated execution of the transactional execution, and whether theoperating system needs to see the interruption.) When the effective PIFCis two, program exception conditions having a transactional executionclass of 1 result in an interruption. A PIFC of 3 is reserved.

Bits 8-11 of the I₂ field (bits 40-43 of the instruction) are reservedand should contain zeros; otherwise, the program may not operatecompatibly in the future.

One embodiment of a format of a Transaction Begin constrained (TBEGINC)instruction is described with reference to FIGS. 3A-3B. In one example,TBEGINC 300 includes an opcode field 302 that includes an opcodespecifying a transaction begin constrained operation; a base field (B₁)304; a displacement field (D₁) 306; and an immediate field (I₂) 308. Thecontents of the general register specified by B₁ 304 are added to D₁ 306to obtain the first operand address. However, with the Transaction Beginconstrained instruction, the first operand address is not used to accessstorage. Instead, the B₁ field of the instruction includes zeros;otherwise, a specification exception is recognized.

In one embodiment, the I₂ field includes various controls, an example ofwhich is depicted in FIG. 3B.

The bits of the I₂ field are defined as follows, in one example:

General Register Save Mask (GRSM) 310:

Bits 0-7 of the I₂ field contain the general register save mask (GRSM).Each bit of the GRSM represents an even-odd pair of general registers,where bit 0 represents registers 0 and 1, bit 1 represents registers 2and 3, and so forth. When a bit in the GRSM is zero, the correspondingregister pair is not saved. When a bit in the GRSM is one, thecorresponding register pair is saved in a model-dependent location thatis not directly accessible by the program.

If the transaction aborts, saved register pairs are restored to theircontents when the outermost TRANSACTION BEGIN instruction was executed.The contents of all other (unsaved) general registers are not restoredwhen a constrained transaction aborts.

When TBEGINC is used to continue execution in the nonconstrainedtransaction execution mode, the general register save mask is ignored.

Allow AR Modification (A) 312:

The A control, bit 12 of the I₂ field, controls whether the transactionis allowed to modify an access register. The effectiveallow-AR-modification control is the logical AND of the A control in theTBEGINC instruction for the current nesting level and for any outerTBEGIN or TBEGINC instructions.

If the effective A control is zero, the transaction will be aborted withabort code 11 (restricted instruction) if an attempt is made to modifyany access register. If the effective A control is one, the transactionwill not be aborted if an access register is modified (absent of anyother abort condition).

Bits 8-11 and 13-15 of the I₂ field (bits 40-43 and 45-47 of theinstruction) are reserved and should contain zeros.

The end of a Transaction Begin instruction is specified by a TRANSACTIONEND (TEND) instruction, a format of which is depicted in FIG. 4. As oneexample, a TEND instruction 400 includes an opcode field 402 thatincludes an opcode specifying a transaction end operation.

A number of terms are used with respect to the transactional executionfacility, and therefore, solely for convenience, a list of terms isprovided below in alphabetical order. In one embodiment, these termshave the following definition:

Abort:

A transaction aborts when it is ended prior to a TRANSACTION ENDinstruction that results in a transaction nesting depth of zero. When atransaction aborts, the following occurs, in one embodiment:

-   -   Transactional store accesses made by any and all levels of the        transaction are discarded (that is, not committed).    -   Non-transactional store accesses made by any and all levels of        the transaction are committed.    -   Registers designated by the general register save mask (GRSM) of        the outermost TRANSACTION BEGIN instruction are restored to        their contents prior to the transactional execution (that is, to        their contents at execution of the outermost TRANSACTION BEGIN        instruction). General registers not designated by the general        register save mask of the outermost TRANSACTION BEGIN        instruction are not restored.    -   Access registers, floating-point registers, and the        floating-point control register are not restored. Any changes        made to these registers during transaction execution are        retained when the transaction aborts.

A transaction may be aborted due to a variety of reasons, includingattempted execution of a restricted instruction, attempted modificationof a restricted resource, transactional conflict, exceeding various CPUresources, any interpretive-execution interception condition, anyinterruption, a TRANSACTION ABORT instruction, and other reasons. Atransaction-abort code provides specific reasons why a transaction maybe aborted.

One example of a format of a TRANSACTION ABORT (TABORT) instruction isdescribed with reference to FIG. 5. As one example, a TABORT instruction500 includes an opcode field 502 that includes an opcode specifying atransaction abort operation; a base field (B₂) 504; and a displacementfield (D₂) 506. When the B₂ field is nonzero, the contents of thegeneral register specified by B₂ 504 are added to D₂ 506 to obtain asecond operand address; otherwise, the second operand address is formedsolely from the D₂ field, and the B₂ field is ignored. The secondoperand address is not used to address data; instead, the address formsthe transaction abort code which is placed in a transaction diagnosticblock during abort processing. Address computation for the secondoperand address follows the rules of address arithmetic: in the 24-bitaddressing mode, bits 0-29 are set to zeros; in the 31-bit addressingmode, bits 0-32 are set to zeros.

Commit:

At the completion of an outermost TRANSACTION END instruction, the CPUcommits the store accesses made by the transaction (i.e., the outermosttransaction and any nested levels) such that they are visible to otherCPUs and the I/O subsystem. As observed by other CPUs and by the I/Osubsystem, all fetch and store accesses made by all nested levels of thetransaction appear to occur as a single concurrent operation when thecommit occurs.

The contents of the general registers, access registers, floating-pointregisters, and the floating-point control register are not modified bythe commit process. Any changes made to these registers duringtransactional execution are retained when the transaction's stores arecommitted.

Conflict:

A transactional access made by one CPU conflicts with either (a) atransactional access or non-transactional access made by another CPU, or(b) the non-transactional access made by the I/O subsystem, if bothaccesses are to any location within the same cache line, and one or moreof the accesses is a store.

A conflict may be detected by a CPU's speculative execution ofinstructions, even though the conflict may not be detected in theconceptual sequence.

Constrained Transaction:

-   -   A constrained transaction is a transaction that executes in the        constrained transactional execution mode and is subject to the        following limitations:    -   A subset of the general instructions is available.    -   A limited number of instructions may be executed.    -   A limited number of storage-operand locations may be accessed.    -   The transaction is limited to a single nesting level.

In the absence of repeated interruptions or conflicts with other CPUs orthe I/O subsystem, a constrained transaction eventually completes, thusan abort-handler routine is not required. Constrained transactions aredescribed in detail below.

When a TRANSACTION BEGIN constrained (TBEGINC) instruction is executedwhile the CPU is already in the nonconstrained transaction executionmode, execution continues as a nested nonconstrained transaction.

Constrained Transactional Execution Mode:

When the transaction nesting depth is zero, and a transaction isinitiated by a TBEGINC instruction, the CPU enters the constrainedtransactional execution mode. While the CPU is in the constrainedtransactional execution mode, the transaction nesting depth is one.

Nested Transaction:

When the TRANSACTION BEGIN instruction is issued while the CPU is in thenonconstrained transactional execution mode, the transaction is nested.

The transactional execution facility uses a model called flattenednesting. In the flattened nesting mode, stores made by an innertransaction are not observable by other CPUs and by the I/O subsystemuntil the outermost transaction commits its stores. Similarly, if atransaction aborts, all nested transactions abort, and all transactionalstores of all nested transactions are discarded.

One example of nested transactions is depicted in FIG. 6. As shown, afirst TBEGIN 600 starts an outermost transaction 601, TBEGIN 602 startsa first nested transaction, and TBEGIN 604 starts a second nestedtransaction. In this example, TBEGIN 604 and TEND 606 define aninnermost transaction 608. When TEND 610 executes, transactional storesare committed 612 for the outermost transaction and all innertransactions.

Nonconstrained Transaction:

A nonconstrained transaction is a transaction that executes in thenonconstrained transactional execution mode. Although a nonconstrainedtransaction is not limited in the manner as a constrained transaction,it may still be aborted due to a variety of causes.

Nonconstrained Transactional Execution Mode:

When a transaction is initiated by the TBEGIN instruction, the CPUenters the nonconstrained transactional execution mode. While the CPU isin the nonconstrained transactional execution mode, the transactionnesting depth may vary from one to the maximum transaction nestingdepth.

Non-Transactional Access:

Non-transactional accesses are storage operand accesses made by the CPUwhen it is not in the transactional execution mode (that is, classicstorage accesses outside of a transaction). Further, accesses made bythe I/O subsystem are non-transactional accesses. Additionally, theNONTRANSACTIONAL STORE instruction may be used to cause anon-transactional store access while the CPU is in the nonconstrainedtransactional execution mode.

One embodiment of a format of a NONTRANSACTIONAL STORE instruction isdescribed with reference to FIG. 7. As one example, a NONTRANSACTIONALSTORE instruction 700 includes a plurality of opcode fields 702 a, 702 bspecifying an opcode that designates a nontransactional store operation;a register field (R₁) 704 specifying a register, the contents of whichare called the first operand; an index field (X₂) 706; a base field (B₂)708; a first displacement field (DL₂) 710; and a second displacementfield (DH₂) 712. The contents of the general registers designated by theX₂ and B₂ fields are added to the contents of a concatenation ofcontents of the DH₂ and DL₂ fields to form the second operand address.When either or both the X₂ or B₂ fields are zero, the correspondingregister does not take part in the addition.

The 64 bit first operand is nontransactionally placed unchanged at thesecond operand location.

The displacement, formed by the concatenation of the DH₂ and DL₂ fields,is treated as a 20-bit signed binary integer.

The second operand is to be aligned on a double word boundary;otherwise, specification exception is recognized and the operation issuppressed.

Outer/Outermost Transaction:

A transaction with a lower-numbered transaction nesting depth is anouter transaction. A transaction with a transaction nesting depth valueof one is the outermost transaction.

An outermost TRANSACTION BEGIN instruction is one that is executed whenthe transaction nesting depth is initially zero. An outermostTRANSACTION END instruction is one that causes the transaction nestingdepth to transition from one to zero. A constrained transaction is theoutermost transaction, in this embodiment.

Program Interruption Filtering:

When a transaction is aborted due to certain program exceptionconditions, the program can optionally prevent the interruption fromoccurring. This technique is called program interruption filtering.Program-interruption filtering is subject to the transactional class ofthe interruption, the effective program interruption filtering controlfrom the TRANSACTION BEGIN instruction, and the transactional executionprogram interruption filtering override in control register 0.

Transaction:

A transaction includes the storage-operand accesses made, and selectedgeneral registers altered, while the CPU is in the transaction executionmode. For a nonconstrained transaction, storage-operand accesses mayinclude both transactional accesses and non-transactional accesses. Fora constrained transaction, storage-operand accesses are limited totransactional accesses. As observed by other CPUs and by the I/Osubsystem, all storage-operand accesses made by the CPU while in thetransaction execution mode appear to occur as a single concurrentoperation. If a transaction is aborted, transactional store accesses arediscarded, and any registers designated by the general register savemask of the outermost TRANSACTION BEGIN instruction are restored totheir contents prior to transactional execution.

Transactional Accesses:

Transactional accesses are storage operand accesses made while the CPUis in the transactional execution mode, with the exception of accessesmade by the NONTRANSACTIONAL STORE instruction.

Transactional Execution Mode:

The term transactional execution mode (a.k.a., transaction executionmode) describes the common operation of both the nonconstrained and theconstrained transactional execution modes. Thus, when the operation isdescribed, the terms nonconstrained and constrained are used to qualifythe transactional execution mode.

When the transaction nesting depth is zero, the CPU is not in thetransactional execution mode (also called the non-transactionalexecution mode).

As observed by the CPU, fetches and stores made in the transactionalexecution mode are no different than those made while not in thetransactional execution mode.

In one embodiment of the z/Architecture, the transactional executionfacility is under the control of bits 8-9 of control register 0, bits61-63 of control register 2, the transaction nesting depth, thetransaction diagnostic block address, and the transaction abort programstatus word (PSW).

Following an initial CPU reset, the contents of bit positions 8-9 ofcontrol register 0, bit positions 62-63 of control register 2, and thetransaction nesting depth are set to zero. When the transactionalexecution control, bit 8 of control register 0, is zero, the CPU cannotbe placed into the transactional execution mode.

Further details regarding the various controls are described below.

As indicated, the transactional execution facility is controlled by twobits in control register zero and three bits in control register two.For instance:

Control Register 0 Bits:

The bit assignments are as follows, in one embodiment:

-   -   Transactional Execution Control (TXC): Bit 8 of control register        zero is the transactional execution control. This bit provides a        mechanism whereby the control program (e.g., operating system)        can indicate whether or not the transactional execution facility        is usable by the program. Bit 8 is to be one to successfully        enter the transactional execution mode.    -   When bit 8 of control register 0 is zero, attempted execution of        the EXTRACT TRANSACTION NESTING DEPTH, TRANSACTION BEGIN and        TRANSACTION END instructions results in a special operation        execution.    -   One embodiment of a format of an EXTRACT TRANSACTION NESTING        DEPTH instruction is described with reference to FIG. 8. As one        example, an EXTRACT TRANSACTION NESTING DEPTH instruction 800        includes an opcode field 802 specifying an opcode that indicates        the extract transaction nesting depth operation; and a register        field R₁ 804 that designates a general register.    -   The current transaction nesting depth is placed in bits 48-63 of        general register R₁. Bits 0-31 of the register remain unchanged,        and bits 32-47 of the register are set to zero.    -   In a further embodiment, the maximum transaction nesting depth        is also placed in general register R₁, such as in bits 16-31.    -   Transaction Execution Program Interruption Filtering Override        (PIFO): Bit 9 of control register zero is the transactional        execution program interruption filtering override. This bit        provides a mechanism by which the control program can ensure        that any program exception condition that occurs while the CPU        is in the transactional execution mode results in an        interruption, regardless of the effective program interruption        filtering control specified or implied by the TRANSACTION BEGIN        instruction(s).

Control Register 2 Bits:

The assignments are as follows, in one embodiment:

-   -   Transaction Diagnostic Scope (TDS): Bit 61 of control register 2        controls the applicability of the transaction diagnosis control        (TDC) in bits 62-63 of the register, as follows:    -   TDS    -   Value Meaning    -   0 The TDC applies regardless of whether the CPU is in the        problem or supervisor state.    -   1 The TDC applies only when the CPU is in the problem state.        When the CPU is in the supervisor state, processing is as if the        TDC contained zero.    -   Transaction Diagnostic Control (TDC): Bits 62-63 of control        register 2 are a 2-bit unsigned integer that may be used to        cause transactions to be randomly aborted for diagnostic        purposes. The encoding of the TDC is as follows, in one example:    -   TDC    -   Value Meaning    -   0 Normal operation; transactions are not aborted as a result of        the TDC.    -   1 Abort every transaction at a random instruction, but before        execution of the outermost TRANSACTION END instruction.    -   2 Abort random transactions at a random instruction.    -   3 Reserved

When a transaction is aborted due to a nonzero TDC, then either of thefollowing may occur:

-   -   The abort code is set to any of the codes 7-11, 13-16, or 255,        with the value of the code randomly chosen by the CPU; the        condition code is set corresponding to the abort code. Abort        codes are further described below.    -   For a nonconstrained transaction, the condition code is set to        one. In this case, the abort code is not applicable.

It is model dependent whether TDC value 1 is implemented. If notimplemented, a value of 1 acts as if 2 was specified.

For a constrained transaction, a TDC value of 1 is treated as if a TDCvalue of 2 was specified.

If a TDC value of 3 is specified, the results are unpredictable.

Transaction Diagnostic Block Address (TDBA)

A valid transaction diagnostic block address (TDBA) is set from thefirst operand address of the outermost TRANSACTION BEGIN (TBEGIN)instruction when the B₁ field of the instruction is nonzero. When theCPU is in the primary space or access register mode, the TDBA designatesa location in the primary address space. When the CPU is in thesecondary space, or home space mode, the TDBA designates a location inthe secondary or home address space, respectively. When DAT (DynamicAddress Translation) is off, the TDBA designates a location in realstorage.

The TDBA is used by the CPU to locate the transaction diagnosticblock—called the TBEGIN-specified TDB—if the transaction is subsequentlyaborted. The rightmost three bits of the TDBA are zero, meaning that theTBEGIN-specified TDB is on a doubleword boundary.

When the B₁ field of an outermost TRANSACTION BEGIN (TBEGIN) instructionis zero, the transactional diagnostic block address is invalid, and noTBEGIN-specified TDB is stored if the transaction is subsequentlyaborted.

Transaction Abort PSW (TAPSW)

During execution of the TRANSACTION BEGIN (TBEGIN) instruction when thenesting depth is initially zero, the transaction abort PSW is set to thecontents of the current PSW; and the instruction address of thetransaction abort PSW designates the next sequential instruction (thatis, the instruction following the outermost TBEGIN). During execution ofthe TRANSACTION BEGIN constrained (TBEGINC) instruction when the nestingdepth is initially zero, the transaction abort PSW is set to thecontents of the current PSW, except that the instruction address of thetransaction abort PSW designates the TBEGINC instruction (rather thanthe next sequential instruction following the TBEGINC).

When a transaction is aborted, the condition code in the transactionabort PSW is replaced with a code indicating the severity of the abortcondition. Subsequently, if the transaction was aborted due to causesthat do not result in an interruption, the PSW is loaded from thetransaction abort PSW; if the transaction was aborted due to causes thatresult in an interruption, the transaction abort PSW is stored as theinterruption old PSW.

The transaction abort PSW is not altered during the execution of anyinner TRANSACTION BEGIN instruction.

Transaction Nesting Depth (TND)

The transaction nesting depth is, for instance, a 16-bit unsigned valuethat is incremented each time a TRANSACTION BEGIN instruction iscompleted with condition code 0 and decremented each time a TRANSACTIONEND instruction is completed. The transaction nesting depth is reset tozero when a transaction is aborted or by CPU reset.

In one embodiment, a maximum TND of 15 is implemented.

In one implementation, when the CPU is in the constrained transactionalexecution mode, the transaction nesting depth is one. Additionally,although the maximum TND can be represented as a 4-bit value, the TND isdefined to be a 16-bit value to facilitate its inspection in thetransaction diagnostic block.

Transaction Diagnostic Block (TDB)

When a transaction is aborted, various status information may be savedin a transaction diagnostic block (TDB), as follows:

-   -   1. TBEGIN-specified TDB: For a nonconstrained transaction, when        the B₁ field of the outermost TBEGIN instruction is nonzero, the        first operand address of the instruction designates the        TBEGIN-specified TDB. This is an application program specified        location that may be examined by the application's abort        handler.    -   2. Program-Interruption (PI) TDB: If a nonconstrained        transaction is aborted due to a non-filtered program exception        condition, or if a constrained transaction is aborted due to any        program exception condition (that is, any condition that results        in a program interruption being recognized), the PI-TDB is        stored into locations in the prefix area. This is available for        the operating system to inspect and log out in any diagnostic        reporting that it may provide.    -   3. Interception TDB: If the transaction is aborted due to any        program exception condition that results in interception (that        is, the condition causes interpretive execution to end and        control to return to the host program), a TDB is stored into a        location specified in the state description block for the guest        operating system.

The TBEGIN-specified TDB is only stored, in one embodiment, when the TDBaddress is valid (that is, when the outermost TBEGIN instruction's B₁field is nonzero).

For aborts due to unfiltered program exception conditions, only one ofeither the PI-TDB or Interception TDB will be stored. Thus, there may bezero, one, or two TDBs stored for an abort.

Further details regarding one example of each of the TDBs are describedbelow:

TBEGIN-Specified TDB:

The 256-byte location specified by a valid transaction diagnostic blockaddress. When the transaction diagnostic block address is valid, theTBEGIN-specified TDB is stored on a transaction abort. TheTBEGIN-specified TDB is subject to all storage protection mechanismsthat are in effect at the execution of the outermost TRANSACTION BEGINinstruction. A PER (Program Event Recording) storage alteration eventfor any portion of the TBEGIN-specified TDB is detected during theexecution of the outermost TBEGIN, not during the transaction abortprocessing.

One purpose of PER is to assist in debugging programs. It permits theprogram to be alerted to the following types of events, as examples:

-   -   Execution of a successful branch instruction. The option is        provided of having an event occur only when the branch target        location is within the designated storage area.    -   Fetching of an instruction from the designated storage area.    -   Alteration of the contents of the designated storage area. The        option is provided of having an event occur only when the        storage area is within designated address spaces.    -   Execution of a STORE USING REAL ADDRESS instruction.    -   Execution of the TRANSACTION END instruction.

The program can selectively specify that one or more of the above typesof events be recognized, except that the event for STORE USING REALADDRESS can be specified only along with the storage alteration event.The information concerning a PER event is provided to the program bymeans of a program interruption, with the cause of the interruptionbeing identified in the interruption code.

When the transaction diagnostic block address is not valid, aTBEGIN-specified TDB is not stored.

Program-Interruption TDB:

Real locations 6,144-6,399 (1800-18FF hex). The program interruption TDBis stored when a transaction is aborted due to program interruption.When a transaction is aborted due to other causes, the contents of theprogram interruption TDB are unpredictable.

The program interruption TDB is not subject to any protection mechanism.PER storage alteration events are not detected for the programinterruption TDB when it is stored during a program interruption.

Interception TDB:

The 256-byte host real location specified by locations 488-495 of thestate description. The interception TDB is stored when an abortedtransaction results in a guest program interruption interception (thatis, interception code 8). When a transaction is aborted due to othercauses, the contents of the interception TDB are unpredictable. Theinterception TDB is not subject to any protection mechanism.

As depicted in FIG. 9, the fields of a transaction diagnostic block 900are as follows, in one embodiment:

Format 902: Byte 0 contains a validity and format indication, asfollows:

-   -   Value Meaning    -   0 The remaining fields of the TDB are unpredictable.    -   1 A format-1 TDB, the remaining fields of which are described        below.    -   2-255 Reserved

A TDB in which the format field is zero is referred to as a null TDB.

Flags 904: Byte 1 contains various indications, as follows:

-   -   Conflict Token Validity (CTV): When a transaction is aborted due        to a fetch or store conflict (that is, abort codes 9 or 10,        respectively), bit 0 of byte 1 is the conflict token validity        indication. When the CTV indication is one, the conflict token        910 in bytes 16-23 of the TDB contain the logical address at        which the conflict was detected. When the CTV indication is        zero, bytes 16-23 of the TDB are unpredictable.    -   When a transaction is aborted due to any reason other than a        fetch or store conflict, bit 0 of byte 1 is stored as zero.    -   Constrained-Transaction Indication (CTI): When the CPU is in the        constrained transactional execution mode, bit 1 of byte 1 is set        to one. When the CPU is in the nonconstrained transactional        execution mode, bit 1 of byte 1 is set to zero.    -   Reserved: Bits 2-7 of byte 1 are reserved, and stored as zeros.

Transaction Nesting Depth (TND) 906: Bytes 6-7 contain the transactionnesting depth when the transaction was aborted.

Transaction Abort Code (TAC) 908: Bytes 8-15 contain a 64-bit unsignedtransaction abort code. Each code point indicates a reason for atransaction being aborted.

It is model dependent whether the transaction abort code is stored inthe program interruption TDB when a transaction is aborted due toconditions other than a program interruption.

Conflict Token 910: For transactions that are aborted due to fetch orstore conflict (that is, abort codes 9 and 10, respectively), bytes16-23 contain the logical address of the storage location at which theconflict was detected. The conflict token is meaningful when the CTVbit, bit 0 of byte 1, is one.

When the CTV bit is zero, bytes 16-23 are unpredictable.

Because of speculative execution by the CPU, the conflict token maydesignate a storage location that would not necessarily be accessed bythe transaction's conceptual execution sequence.

Aborted Transaction Instruction Address (ATIA) 912: Bytes 24-31 containan instruction address that identifies the instruction that wasexecuting when an abort was detected. When a transaction is aborted dueto abort codes 2, 5, 6, 11, 13, or 256 or higher, or when a transactionis aborted due to abort codes 4 or 13 and the program exceptioncondition is nullifying, the ATIA points directly to the instructionthat was being executed. When a transaction is aborted due to abortcodes 4 or 12, and the program exception condition is not nullifying,the ATIA points past the instruction that was being executed.

When a transaction is aborted due to abort codes 7-10, 14-16, or 255,the ATIA does not necessarily indicate the exact instruction causing theabort, but may point to an earlier or later instruction within thetransaction.

If a transaction is aborted due to an instruction that is the target ofan execute-type instruction, the ATIA identifies the execute-typeinstruction, either pointing to the instruction or past it, depending onthe abort code as described above. The ATIA does not indicate the targetof the execute-type instruction.

The ATIA is subject to the addressing mode when the transaction isaborted. In the 24-bit addressing mode, bits 0-40 of the field containzeros. In the 31-bit addressing mode, bits 0-32 of the field containzeros.

It is model dependent whether the aborted transaction instructionaddress is stored in the program interruption TDB when a transaction isaborted due to conditions other than a program interruption.

When a transaction is aborted due to abort code 4 or 12, and the programexception condition is not nullifying, the ATIA does not point to theinstruction causing the abort. By subtracting the number of halfwordsindicated by the interruption length code (ILC) from the ATIA, theinstruction causing the abort can be identified in conditions that aresuppressing or terminating, or for non-PER events that are completing.When a transaction is aborted due to a PER event, and no other programexception condition is present, the ATIA is unpredictable.

When the transaction diagnostic block address is valid, the ILC may beexamined in program interruption identification (PIID) in bytes 36-39 ofthe TBEGIN-specified TDB. When filtering does not apply, the ILC may beexamined in the PIID at location 140-143 in real storage.

Exception Access Identification (EAID) 914: For transactions that areaborted due to certain filtered program exception conditions, byte 32 ofthe TBEGIN-specified TDB contains the exception access identification.In one example of the z/Architecture, the format of the EAID, and thecases for which it is stored, are the same as those described in reallocation 160 when the exception condition results in an interruption, asdescribed in the above-incorporated by reference Principles ofOperation.

For transactions that are aborted for other reasons, including anyexception conditions that result in a program interruption, byte 32 isunpredictable. Byte 32 is unpredictable in the program interruption TDB.

This field is stored only in the TDB designated by the transactiondiagnostic block address; otherwise, the field is reserved. The EAID isstored only for access list controlled or DAT protection, ASCE-type,page translation, region first translation, region second translation,region third translation, and segment translation program exceptionconditions.

Data Exception Code (DXC) 916: For transactions that are aborted due tofiltered data exception program exception conditions, byte 33 of theTBEGIN specified TDB contains the data exception code. In one example ofthe z/Architecture, the format of the DXC, and the cases for which it isstored, are the same as those described in real location 147 when theexception condition results in an interruption, as described in theabove-incorporated by reference Principles of Operation. In one example,location 147 includes the DXC.

For transactions that are aborted for other reasons, including anyexception conditions that result in a program interruption, byte 33 isunpredictable. Byte 33 is unpredictable in the program interruption TDB.

This field is stored only in the TDB designated by the transactiondiagnostic block address; otherwise, the field is reserved. The DXC isstored only for data program exception conditions.

Program Interruption Identification (PIID) 918: For transactions thatare aborted due to filtered program interruption conditions, bytes 36-39of the TBEGIN-specified TDB contain the program exceptionidentification. In one example of the z/Architecture, the format of thePIID is the same as that described in real locations 140-143 when thecondition results in an interruption (as described in theabove-incorporated by reference Principles of Operation), except thatthe instruction length code in bits 13-14 of the PIID is respective tothe instruction at which the exception condition was detected.

For transactions that are aborted for other reasons, including exceptionconditions that result in a program interruption, bytes 36-39 areunpredictable. Bytes 36-39 are unpredictable in the program interruptionTDB.

This field is stored only in the TDB designated by the transactiondiagnostic block address; otherwise, the field is reserved. The programinterruption identification is only stored for program exceptionconditions.

Translation Exception Identification (TEID) 920: For transactions thatare aborted due to any of the following filtered program exceptionconditions, bytes 40-47 of the TBEGIN-specified TDB contain thetranslation exception identification.

-   -   Access list controlled or DAT protection    -   ASCE-type    -   Page translation    -   Region-first translation    -   Region-second translation    -   Region-third translation    -   Segment translation exception

In one example of the z/Architecture, the format of the TEID is the sameas that described in real locations 168-175 when the condition resultsin an interruption, as described in the above-incorporated by referencePrinciples of Operation.

For transactions that are aborted for other reasons, including exceptionconditions that result in a program interruption, bytes 40-47 areunpredictable. Bytes 40-47 are unpredictable in the program interruptionTDB.

This field is stored only in the TDB designated by the transactiondiagnostic block address; otherwise, the field is reserved.

Breaking Event Address 922: For transactions that are aborted due tofiltered program exception conditions, bytes 48-55 of theTBEGIN-specified TDB contain the breaking event address. In one exampleof the z/Architecture, the format of the breaking event address is thesame as that described in real locations 272-279 when the conditionresults in an interruption, as described in the above-incorporated byreference Principles of Operation.

For transactions that are aborted for other reasons, including exceptionconditions that result in a program interruption, bytes 48-55 areunpredictable. Bytes 48-55 are unpredictable in the program interruptionTDB.

This field is stored only in the TDB designated by the transactiondiagnostic block address; otherwise, the field is reserved.

Further details relating to breaking events are described below.

In one embodiment of the z/Architecture, when the PER-3 facility isinstalled, it provides the program with the address of the lastinstruction to cause a break in the sequential execution of the CPU.Breaking event address recording can be used as a debugging assist forwild branch detection. This facility provides, for instance, a 64-bitregister in the CPU, called the breaking event address register. Eachtime an instruction other than TRANSACTION ABORT causes a break in thesequential instruction execution (that is, the instruction address inthe PSW is replaced, rather than incremented by the length of theinstruction), the address of that instruction is placed in the breakingevent address register. Whenever a program interruption occurs, whetheror not PER is indicated, the current contents of the breaking eventaddress register are placed in real storage locations 272-279.

If the instruction causing the breaking event is the target of anexecute-type instruction (EXECUTE or EXECUTE RELATIVE LONG), then theinstruction address used to fetch the execute-type instruction is placedin the breaking event address register.

In one embodiment of the z/Architecture, a breaking event is consideredto occur whenever one of the following instructions causes branching:BRANCH AND LINK (BAL, BALR); BRANCH AND SAVE (BAS, BASR); BRANCH ANDSAVE AND SET MODE (BASSM); BRANCH AND SET MODE (BSM); BRANCH AND STACK(BAKR); BRANCH ON CONDITION (BC, BCR); BRANCH ON COUNT (BCT, BCTR, BCTG,BCTGR); BRANCH ON INDEX HIGH (BXH, BXHG); BRANCH ON INDEX LOW OR EQUAL(BXLE, BXLEG); BRANCH RELATIVE ON CONDITION (BRC); BRANCH RELATIVE ONCONDITION LONG (BRCL); BRANCH RELATIVE ON COUNT (BRCT, BRCTG); BRANCHRELATIVE ON INDEX HIGH (BRXH, BRXHG); BRANCH RELATIVE ON INDEX LOW OREQUAL (BRXLE, BRXLG); COMPARE AND BRANCH (CRB, CGRB); COMPARE AND BRANCHRELATIVE (CRJ, CGRJ); COMPARE IMMEDIATE AND BRANCH (CIB, CGIB); COMPAREIMMEDIATE AND BRANCH RELATIVE (CIJ, CGIJ); COMPARE LOGICAL AND BRANCH(CLRB, CLGRB); COMPARE LOGICAL AND BRANCH RELATIVE (CLRJ, CLGRJ);COMPARE LOGICAL IMMEDIATE AND BRANCH (CLIB, CLGIB); and COMPARE LOGICALIMMEDIATE AND BRANCH RELATIVE (CLIJ, CLGIJ).

A breaking event is also considered to occur whenever one of thefollowing instructions completes: BRANCH AND SET AUTHORITY (BSA); BRANCHIN SUBSPACE GROUP (BSG); BRANCH RELATIVE AND SAVE (BRAS); BRANCHRELATIVE AND SAVE LONG (BRASL); LOAD PSW (LPSW); LOAD PSW EXTENDED(LPSWE); PROGRAM CALL (PC); PROGRAM RETURN (PR); PROGRAM TRANSFER (PT);PROGRAM TRANSFER WITH INSTANCE (PTI); RESUME PROGRAM (RP); and TRAP(TRAP2, TRAP4).

A breaking event is not considered to occur as a result of a transactionbeing aborted (either implicitly or as a result of the TRANSACTION ABORTinstruction).

Model Dependent Diagnostic Information 924: Bytes 112-127 contain modeldependent diagnostic information.

For all abort codes except 12 (filtered program interruption), the modeldependent diagnostic information is saved in each TDB that is stored.

In one embodiment, the model dependent diagnostic information includesthe following:

-   -   Bytes 112-119 contain a vector of 64 bits called the        transactional execution branch indications (TXBI). Each of the        first 63 bits of the vector indicates the results of executing a        branching instruction while the CPU was in the transactional        execution mode, as follows:    -   Value Meaning    -   0 The instruction completed without branching.    -   1 The instruction completed with branching.    -   Bit 0 represents the result of the first such branching        instruction, bit 1 represents the result of the second such        instruction, and so forth.    -   If fewer than 63 branching instructions were executed while the        CPU was in the transactional execution mode, the rightmost bits        that do not correspond to branching instructions are set to        zeros (including bit 63). When more than 63 branching        instructions were executed, bit 63 of the TXBI is set to one.    -   Bits in the TXBI are set by instructions which are capable of        causing a breaking event, as listed above, except for the        following:        -   Any restricted instruction does not cause a bit to be set in            the TXBI.        -   For instructions of, for instance, the z/Architecture, when            the M₁ field of the BRANCH ON CONDITION, BRANCH RELATIVE ON            CONDITION, or BRANCH RELATIVE ON CONDITION LONG instruction            is zero, or when the R₂ field of the following instructions            is zero, it is model dependent whether the execution of the            instruction causes a bit to be set in the TXBI.            -   BRANCH AND LINK (BALR); BRANCH AND SAVE (BASR); BRANCH                AND SAVE AND SET MODE (BASSM); BRANCH AND SET MODE                (BSM); BRANCH ON CONDITION (BCR); and BRANCH ON COUNT                (BCTR, BCTGR)    -   For abort conditions that were caused by a host access        exception, bit position 0 of byte 127 is set to one. For all        other abort conditions, bit position 0 of byte 127 is set to        zero.    -   For abort conditions that were detected by the load/store unit        (LSU), the rightmost five bits of byte 127 contain an indication        of the cause. For abort conditions that were not detected by the        LSU, byte 127 is reserved.

General Registers 930: Bytes 128-255 contain the contents of generalregisters 0-15 at the time the transaction was aborted. The registersare stored in ascending order, beginning with general register 0 inbytes 128-135, general register 1 in bytes 136-143, and so forth.

Reserved: All other fields are reserved. Unless indicated otherwise, thecontents of reserved fields are unpredictable.

As observed by other CPUs and the I/O subsystem, storing of the TDB(s)during a transaction abort is a multiple access reference occurringafter any non-transactional stores.

A transaction may be aborted due to causes that are outside the scope ofthe immediate configuration in which it executes. For example, transientevents recognized by a hypervisor (such as LPAR or z/VM) may cause atransaction to be aborted.

The information provided in the transaction diagnostic block is intendedfor diagnostic purposes and is substantially correct. However, becausean abort may have been caused by an event outside the scope of theimmediate configuration, information such as the abort code or programinterruption identification may not accurately reflect conditions withinthe configuration, and thus, should not be used in determining programaction.

In addition to the diagnostic information saved in the TDB, when atransaction is aborted due to any data exception program exceptioncondition and both the AFP register control, bit 45 of control register0, and the effective allow floating point operation control (F) are one,the data exception code (DXC) is placed into byte 2 of the floatingpoint control register (FPCR), regardless of whether filtering appliesto the program exception condition. When a transaction is aborted, andeither or both the AFP register control or effective allow floatingpoint operation control are zero, the DXC is not placed into the FPCR.

In one embodiment, as indicated herein, when the transactional executionfacility is installed, the following general instructions are provided.

-   -   EXTRACT TRANSACTION NESTING DEPTH    -   NONTRANSACTIONAL STORE    -   TRANSACTION ABORT    -   TRANSACTION BEGIN    -   TRANSACTION END

When the CPU is in the transactional execution mode, attempted executionof certain instructions is restricted and causes the transaction to beaborted.

When issued in the constrained transactional execution mode, attemptedexecution of restricted instructions may also result in a transactionconstraint program interruption, or may result in execution proceedingas if the transaction was not constrained.

In one example of the z/Architecture, restricted instructions include,as examples, the following non-privileged instructions: COMPARE AND SWAPAND STORE; MODIFY RUNTIME INSTRUMENTATION CONTROLS; PERFORM LOCKEDOPERATION; PREFETCH DATA (RELATIVE LONG), when the code in the M₁ fieldis 6 or 7; STORE CHARACTERS UNDER MASK HIGH, when the M₃ field is zeroand the code in the R₁ field is 6 or 7; STORE FACILITY LIST EXTENDED;STORE RUNTIME INSTRUMENTATION CONTROLS; SUPERVISOR CALL; and TESTRUNTIME INSTRUMENTATION CONTROLS.

In the above list, COMPARE AND SWAP AND STORE and PERFORM LOCKEDOPERATION are complex instructions which can be more efficientlyimplemented by the use of basic instructions in the TX mode. The casesfor PREFETCH DATA and PREFETCH DATA RELATIVE LONG are restricted as thecodes of 6 and 7 release a cache line, necessitating the commitment ofthe data potentially prior to the completion of a transaction.SUPERVISOR CALL is restricted as it causes an interruption (which causesa transaction to be aborted).

Under the conditions listed below, the following instructions arerestricted:

-   -   BRANCH AND LINK (BALR), BRANCH AND SAVE (BASR), and BRANCH AND        SAVE AND SET MODE, when the R₂ field of the instruction is        nonzero and branch tracing is enabled.    -   BRANCH AND SAVE AND SET MODE and BRANCH AND SET MODE, when the        R₂ field is nonzero and mode tracing is enabled; SET ADDRESSING        MODE, when mode tracing is enabled.    -   MONITOR CALL, when a monitor event condition is recognized.

The above list includes instructions that may form trace entries. Ifthese instructions were allowed to execute transactionally and formedtrace entries, and the transaction subsequently aborted, the trace tablepointer in control register 12 would be advanced, but the stores to thetrace table would be discarded. This would leave an inconsistent gap inthe trace table; thus, the instructions are restricted in the caseswhere they would form trace entries.

When the CPU is in the transactional execution mode, it is modeldependent whether the following instructions are restricted: CIPHERMESSAGE; CIPHER MESSAGE WITH CFB; CIPHER MESSAGE WITH CHAINING; CIPHERMESSAGE WITH COUNTER; CIPHER MESSAGE WITH OFB; COMPRESSION CALL; COMPUTEINTERMEDIATE MESSAGE DIGEST; COMPUTE LAST MESSAGE DIGEST; COMPUTEMESSAGE AUTHENTICATION CODE; CONVERT UNICODE-16 TO UNICODE-32; CONVERTUNICODE-16 TO UNICODE-8; CONVERT UNICODE-32 TO UNICODE-16; CONVERTUNICODE-32 TO UNICODE-8; CONVERT UNICODE-8 TO UNICODE-16; CONVERTUNICODE-8 TO UNICODE-32; PERFORM CRYPTOGRAPHIC COMPUTATION; RUNTIMEINSTRUMENTATION OFF; and RUNTIME INSTRUMENTATION ON.

Each of the above instructions is either currently implemented by thehardware coprocessor, or has been in past machines, and thus, isconsidered restricted.

When the effective allow AR modification (A) control is zero, thefollowing instructions are restricted: COPY ACCESS; LOAD ACCESSMULTIPLE; LOAD ADDRESS EXTENDED; and SET ACCESS.

Each of the above instructions causes the contents of an access registerto be modified. If the A control in the TRANSACTION BEGIN instruction iszero, then the program has explicitly indicated that access registermodification is not to be allowed.

When the effective allow floating point operation (F) control is zero,floating point instructions are restricted.

Under certain circumstances, the following instructions may berestricted: EXTRACT CPU TIME; EXTRACT PSW; STORE CLOCK; STORE CLOCKEXTENDED; and STORE CLOCK FAST.

Each of the above instructions is subject to an interception control inthe interpretative execution state description. If the hypervisor hasset the interception control for these instructions, then theirexecution may be prolonged due to hypervisor implementation; thus, theyare considered restricted if an interception occurs.

When a nonconstrained transaction is aborted because of the attemptedexecution of a restricted instruction, the transaction abort code in thetransaction diagnostic block is set to 11 (restricted instruction), andthe condition code is set to 3, except as follows: when a nonconstrainedtransaction is aborted due to the attempted execution of an instructionthat would otherwise result in a privileged operation exception, it isunpredictable whether the abort code is set to 11 (restrictedinstruction) or 4 (unfiltered program interruption resulting from therecognition of the privileged operation program interruption). When anonconstrained transaction is aborted due to the attempted execution ofPREFETCH DATA (RELATIVE LONG) when the code in the M₁ field is 6 or 7 orSTORE CHARACTERS UNDER MASK HIGH when the M₃ field is zero and the codein the R₁ field is 6 or 7, it is unpredictable whether the abort code isset to 11 (restricted instruction) or 16 (cache other). When anonconstrained transaction is aborted due to the attempted execution ofMONITOR CALL, and both a monitor event condition and a specificationexception condition are present it is unpredictable whether the abortcode is set to 11 or 4, or, if the program interruption is filtered, 12.

Additional instructions may be restricted in a constrained transaction.Although these instructions are not currently defined to be restrictedin a nonconstrained transaction, they may be restricted under certaincircumstances in a nonconstrained transaction on future processors.

Certain restricted instructions may be allowed in the transactionalexecution mode on future processors. Therefore, the program should notrely on the transaction being aborted due to the attempted execution ofa restricted instruction. The TRANSACTION ABORT instruction should beused to reliably cause a transaction to be aborted.

In a nonconstrained transaction, the program should provide analternative non-transactional code path to accommodate a transactionthat aborts due to a restricted instruction.

In operation, when the transaction nesting depth is zero, execution ofthe TRANSACTION BEGIN (TBEGIN) instruction resulting in condition codezero causes the CPU to enter the nonconstrained transactional executionmode. When the transaction nesting depth is zero, execution of theTRANSACTION BEGIN constrained (TBEGINC) instruction resulting incondition code zero causes the CPU to enter the constrainedtransactional execution mode.

Except where explicitly noted otherwise, all rules that apply fornon-transactional execution also apply to transactional execution. Beloware additional characteristics of processing while the CPU is in thetransactional execution mode.

When the CPU is in the nonconstrained transactional execution mode,execution of the TRANSACTION BEGIN instruction resulting in conditioncode zero causes the CPU to remain in the nonconstrained transactionalexecution mode.

As observed by the CPU, fetches and stores made in the transactionexecution mode are no different than those made while not in thetransactional execution mode. As observed by other CPUs and by the I/Osubsystem, all storage operand accesses made while a CPU is in thetransactional execution mode appear to be a single block concurrentaccess. That is, the accesses to all bytes within a halfword, word,doubleword, or quadword are specified to appear to be block concurrentas observed by other CPUs and I/O (e.g., channel) programs. Thehalfword, word, doubleword, or quadword is referred to in this sectionas block. When a fetch-type reference is specified to appear to beconcurrent within a block, no store access to the block by another CPUor I/O program is permitted during the time that bytes contained in theblock are being fetched. When a store-type reference is specified toappear to be concurrent within a block, no access to the block, eitherfetch or store, is permitted by another CPU or I/O program during thetime that the bytes within the block are being stored.

Storage accesses for instruction and DAT and ART (Access Register Table)table fetches follow the non-transactional rules.

The CPU leaves the transactional execution mode normally by means of aTRANSACTION END instruction that causes the transaction nesting depth totransition to zero, in which case, the transaction completes.

When the CPU leaves the transactional execution mode by means of thecompletion of a TRANSACTION END instruction, all stores made while inthe transactional execution mode are committed; that is, the storesappear to occur as a single block-concurrent operation as observed byother CPUs and by the I/O subsystem.

A transaction may be implicitly aborted for a variety of causes, or itmay be explicitly aborted by the TRANSACTION ABORT instruction. Examplepossible causes of a transaction abort, the corresponding abort code,and the condition code that is placed into the transaction abort PSW aredescribed below.

External Interruption: The transaction abort code is set to 2, and thecondition code in the transaction abort PSW is set to 2. The transactionabort PSW is stored as the external old PSW as a part of externalinterruption processing.

Program Interruption (Unfiltered): A program exception condition thatresults in an interruption (that is, an unfiltered condition) causes thetransaction to be aborted with code 4. The condition code in thetransaction abort PSW is set specific to the program interruption code.The transaction abort PSW is stored as the program old PSW as a part ofprogram interruption processing.

An instruction that would otherwise result in a transaction beingaborted due to an operation exception may yield alternate results: for anonconstrained transaction, the transaction may instead abort with abortcode 11 (restricted instruction); for a constrained transaction, atransaction constraint program interruption may be recognized instead ofthe operation exception.

When a PER (Program Event Recording) event is recognized in conjunctionwith any other unfiltered program exception condition, the conditioncode is set to 3.

Machine Check Interruption: The transaction abort code is set to 5, andthe condition code in the transaction abort PSW is set to 2. Thetransaction abort PSW is stored as the machine check old PSW as a partof machine check interruption processing.

I/O Interruption: The transaction abort code is set to 6, and thecondition code in the transaction abort PSW is set to 2. The transactionabort PSW is stored as the I/O old PSW as a part of I/O interruptionprocessing.

Fetch Overflow: A fetch overflow condition is detected when thetransaction attempts to fetch from more locations than the CPU supports.The transaction abort code is set to 7, and the condition code is set toeither 2 or 3.

Store Overflow: A store overflow condition is detected when thetransaction attempts to store to more locations than the CPU supports.The transaction abort code is set to 8, and the condition code is set toeither 2 or 3.

Allowing the condition code to be either 2 or 3 in response to a fetchor store overflow abort allows the CPU to indicate potentially retryablesituations (e.g., condition code 2 indicates re-execution of thetransaction may be productive; while condition code 3 does not recommendre-execution).

Fetch Conflict: A fetch conflict condition is detected when another CPUor the I/O subsystem attempts to store into a location that has beentransactionally fetched by this CPU. The transaction abort code is setto 9, and the condition code is set to 2.

Store Conflict: A store conflict condition is detected when another CPUor the I/O subsystem attempts to access a location that has been storedduring transactional execution by this CPU. The transaction abort codeis set to 10, and the condition code is set to 2.

Restricted Instruction: When the CPU is in the transactional executionmode, attempted execution of a restricted instruction causes thetransaction to be aborted. The transaction abort code is set to 11, andthe condition code is set to 3.

When the CPU is in the constrained transactional execution mode, it isunpredictable whether attempted execution of a restricted instructionresults in a transaction constraint program interruption or an abort dueto a restricted instruction. The transaction is still aborted but theabort code may indicate either cause.

Program Exception Condition (Filtered): A program exception conditionthat does not result in an interruption (that is, a filtered condition)causes the transaction to be aborted with a transaction abort code of12. The condition code is set to 3.

Nesting Depth Exceeded: The nesting depth exceeded condition is detectedwhen the transaction nesting depth is at the maximum allowable value forthe configuration, and a TRANSACTION BEGIN instruction is executed. Thetransaction is aborted with a transaction abort code of 13, and thecondition code is set to 3.

Cache Fetch Related Condition: A condition related to storage locationsfetched by the transaction is detected by the CPU's cache circuitry. Thetransaction is aborted with a transaction abort code of 14, and thecondition code is set to either 2 or 3.

Cache Store Related Condition: A condition related to storage locationsstored by the transaction is detected by the CPU's cache circuitry. Thetransaction is aborted with a transaction abort code of 15, and thecondition code is set to either 2 or 3.

Cache Other Condition: A cache other condition is detected by the CPU'scache circuitry. The transaction is aborted with a transaction abortcode of 16, and the condition code is set to either 2 or 3.

During transactional execution, if the CPU accesses instructions orstorage operands using different logical addresses that are mapped tothe same absolute address, it is model dependent whether the transactionis aborted. If the transaction is aborted due to accesses usingdifferent logical addresses mapped to the same absolute address, abortcode 14, 15, or 16 is set, depending on the condition.

Miscellaneous Condition: A miscellaneous condition is any othercondition recognized by the CPU that causes the transaction to abort.The transaction abort code is set to 255 and the condition code is setto either 2 or 3.

When multiple configurations are executing in the same machine (forexample, logical partitions or virtual machines), a transaction may beaborted due to an external machine check or I/O interruption thatoccurred in a different configuration.

Although examples are provided above, other causes of a transactionabort with corresponding abort codes and condition codes may beprovided. For instance, a cause may be a Restart Interruption, in whichthe transaction abort code is set to 1, and the condition code in thetransaction abort PSW is set to 2. The transaction abort PSW is storedas the restart-old PSW as a part of restart processing. As a furtherexample, a cause may be a Supervisor Call condition, in which the abortcode is set to 3, and the condition code in the transaction abort PSW isset to 3. Other or different examples are also possible.

Notes:

-   1. The miscellaneous condition may result from any of the following:    -   Instructions, such as, in the z/Architecture, COMPARE AND        REPLACE DAT TABLE ENTRY, COMPARE AND SWAP AND PURGE, INVALIDATE        DAT TABLE ENTRY, INVALIDATE PAGE TABLE ENTRY, PERFORM FRAME        MANAGEMENT FUNCTION in which the NQ control is zero and the SK        control is one, SET STORAGE KEY EXTENDED in which the NQ control        is zero, performed by another CPU in the configuration; the        condition code is set to 2.    -   An operator function, such as reset, restart or stop, or the        equivalent SIGNAL PROCESSOR order is performed on the CPU.    -   Any other condition not enumerated above; the condition code is        set to 2 or 3.-   2. The location at which fetch and store conflicts are detected may    be anywhere within the same cache line.-   3. Under certain conditions, the CPU may not be able to distinguish    between similar abort conditions. For example, a fetch or store    overflow may be indistinguishable from a respective fetch or store    conflict.-   4. Speculative execution of multiple instruction paths by the CPU    may result in a transaction being aborted due to conflict or    overflow conditions, even if such conditions do not occur in the    conceptual sequence. While in the constrained transactional    execution mode, the CPU may temporarily inhibit speculative    execution, allowing the transaction to attempt to complete without    detecting such conflicts or overflows speculatively.

Execution of a TRANSACTION ABORT instruction causes the transaction toabort. The transaction abort code is set from the second operandaddress. The condition code is set to either 2 or 3, depending onwhether bit 63 of the second operand address is zero or one,respectively.

FIG. 10 summarizes example abort codes stored in the transactiondiagnostic block, and the corresponding condition code (CC). Thedescription in FIG. 10 illustrates one particular implementation. Otherimplementations and encodings of values are possible.

In one embodiment as mentioned above, the transactional facilityprovides for both constrained transactions and nonconstrainedtransactions, as well as processing associated therewith. Initially,constrained transactions are discussed and then nonconstrainedtransactions

A constrained transaction executes in transactional mode without afall-back path. It is a mode of processing useful for compact functions.In the absence of repeated interruptions or conflicts with other CPUs orthe I/O subsystem (i.e., caused by conditions that will not allow thetransaction to complete successfully), a constrained transaction willeventually complete; thus, an abort handler routine is not required andis not specified. For instance, in the absence of violation of acondition that cannot be addressed (e.g., divide by 0); a condition thatdoes not allow the transaction to complete (e.g., a timer interruptionthat does not allow an instruction to run; a hot I/O; etc.); or aviolation of a restriction or constraint associated with a constrainedtransaction, the transaction will eventually complete.

A constrained transaction is initiated by a TRANSACTION BEGINconstrained (TBEGINC) instruction when the transaction nesting depth isinitially zero. A constrained transaction is subject to the followingconstraints, in one embodiment.

-   -   1. The transaction executes no more than 32 instructions, not        including the TRANSACTION BEGIN constrained (TBEGINC) and        TRANSACTION END instructions.    -   2. All instructions in the transaction are to be within 256        contiguous bytes of storage, including the TRANSACTION BEGIN        constrained (TBEGINC) and any TRANSACTION END instructions.    -   3. In addition to the restricted instructions, the following        restrictions apply to a constrained transaction.        -   a. Instructions are limited to those referred to as General            Instructions, including, for instance, add, subtract,            multiply, divide, shift, rotate, etc.        -   b. Branching instructions are limited to the following (the            instructions listed are of the z/Architecture in one            example):            -   BRANCH RELATIVE ON CONDITION in which the M₁ is nonzero                and the RI₂ field contains a positive value.            -   BRANCH RELATIVE ON CONDITION LONG in which the M₁ field                is nonzero, and the RI₂ field contains a positive value                that does not cause address wraparound.            -   COMPARE AND BRANCH RELATIVE, COMPARE IMMEDIATE AND                BRANCH RELATIVE, COMPARE LOGICAL AND BRANCH RELATIVE,                and COMPARE LOGICAL IMMEDIATE AND BRANCH RELATIVE in                which the M₃ field is nonzero and the RI₄ field contains                a positive value. (That is, only forward branches with                nonzero branch masks.)        -   c. Except for TRANSACTION END and instructions which cause a            specified operand serialization, instructions which cause a            serialization function are restricted.        -   d. Storage-and-storage operations (SS-), and            storage-and-storage operations with an extended opcode            (SSE-) instructions are restricted.        -   e. All of the following general instructions (which are of            the z/Architecture in this example) are restricted:            CHECKSUM; CIPHER MESSAGE; CIPHER MESSAGE WITH CFB; CIPHER            MESSAGE WITH CHAINING; CIPHER MESSAGE WITH COUNTER; CIPHER            MESSAGE WITH OFB; COMPARE AND FORM CODEWORD; COMPARE LOGICAL            LONG; COMPARE LOGICAL LONG EXTENDED; COMPARE LOGICAL LONG            UNICODE; COMPARE LOGICAL STRING; COMPARE UNTIL SUBSTRING            EQUAL; COMPRESSION CALL; COMPUTE INTERMEDIATE MESSAGE            DIGEST; COMPUTE LAST MESSAGE DIGEST; COMPUTE MESSAGE            AUTHENTICATION CODE; CONVERT TO BINARY; CONVERT TO DECIMAL;            CONVERT UNICODE-16 TO UNICODE-32; CONVERT UNICODE-16 TO            UNICODE-8; CONVERT UNICODE-32 TO UNICODE-16; CONVERT            UNICODE-32 TO UNICODE-8; CONVERT UNICODE-8 TO UNICODE-16;            CONVERT UNICODE-8 TO UNICODE-32; DIVIDE; DIVIDE LOGICAL;            DIVIDE SINGLE; EXECUTE; EXECUTE RELATIVE LONG; EXTRACT CACHE            ATTRIBUTE; EXTRACT CPU TIME; EXTRACT PSW; EXTRACT            TRANSACTION NESTING DEPTH; LOAD AND ADD; LOAD AND ADD            LOGICAL; LOAD AND AND; LOAD AND EXCLUSIVE OR; LOAD AND OR;            LOAD PAIR DISJOINT; LOAD PAIR FROM QUADWORD; MONITOR CALL;            MOVE LONG; MOVE LONG EXTENDED; MOVE LONG UNICODE; MOVE            STRING; NON-TRANSACTIONAL STORE; PERFORM CRYPTOGRAPHIC            COMPUTATION; PREFETCH DATA; PREFETCH DATA RELATIVE LONG;            RUNTIME INSTRUMENTATION EMIT; RUNTIME INSTRUMENTATION NEXT;            RUNTIME INSTRUMENTATION OFF; RUNTIME INSTRUMENTATION ON;            SEARCH STRING; SEARCH; STRING UNICODE; SET ADDRESSING MODE;            STORE CHARACTERS UNDER MASK HIGH, when the M₃ field is zero,            and the code in the R₁ field is 6 or 7; STORE CLOCK; STORE            CLOCK EXTENDED; STORE CLOCK FAST; STORE FACILITY LIST            EXTENDED; STORE PAIR TO QUADWORD; TEST ADDRESSING MODE;            TRANSACTION ABORT; TRANSACTION BEGIN (both TBEGIN and            TBEGINC); TRANSLATE AND TEST EXTENDED; TRANSLATE AND TEST            REVERSE EXTENDED; TRANSLATE EXTENDED; TRANSLATE ONE TO ONE;            TRANSLATE ONE TO TWO TRANSLATE TWO TO ONE; and TRANSLATE TWO            TO TWO.    -   4. The transaction's storage operands access no more than four        octowords. Note: LOAD ON CONDITION and STORE ON CONDITION are        considered to reference storage regardless of the condition        code. An octoword is, for instance, a group of 32 consecutive        bytes on a 32 byte boundary.    -   5. The transaction executing on this CPU, or stores by other        CPUs or the I/O subsystem, do not access storage operands in any        4 K-byte blocks that contain the 256 bytes of storage beginning        with the TRANSACTION BEGIN constrained (TBEGINC) instruction.    -   6. The transaction does not access instructions or storage        operands using different logical addresses that are mapped to        the same absolute address.    -   7. Operand references made by the transaction are to be within a        single doubleword, except that for LOAD ACCESS MULTIPLE, LOAD        MULTIPLE, LOAD MULTIPLE HIGH, STORE ACCESS MULTIPLE, STORE        MULTIPLE, and STORE MULTIPLE HIGH, operand references are to be        within a single octoword.

If a constrained transaction violates any of constraints 1-7, listedabove, then either (a) a transaction constraint program interruption isrecognized, or (b) execution proceeds as if the transaction was notconstrained, except that further constraint violations may still resultin a transaction constrained program interruption. It is unpredictablewhich action is taken, and the action taken may differ based on whichconstraint is violated.

In the absence of constraint violations, repeated interruptions, orconflicts with other CPUs or the I/O subsystem, a constrainedtransaction will eventually complete, as described above.

-   -   1. The chance of successfully completing a constrained        transaction improves if the transaction meets the following        criteria:        -   a. The instructions issued are fewer than the maximum of 32.        -   b. The storage operand references are fewer than the maximum            of 4 octowords.        -   c. The storage operand references are on the same cache            line.        -   d. Storage operand references to the same locations occur in            the same order by all transactions.    -   2. A constrained transaction is not necessarily assured of        successfully completing on its first execution. However, if a        constrained transaction that does not violate any of the listed        constraints is aborted, the CPU employs circuitry to ensure that        a repeated execution of the transaction is subsequently        successful.    -   3. Within a constrained transaction, TRANSACTION BEGIN is a        restricted instruction, thus a constrained transaction cannot be        nested.    -   4. Violation of any of constrains 1-7 above by a constrained        transaction may result in a program loop.    -   5. The limitations of a constrained transaction are similar to        those of a compare-and-swap loop. Because of potential        interference from other CPUs and the I/O subsystem, there is no        architectural assurance that a COMPARE AND SWAP instruction will        ever complete with condition code 0. A constrained transaction        may suffer from similar interference in the form of fetch- or        store-conflict aborts or hot interruptions.        -   The CPU employs fairness algorithms to ensure that, in the            absence of any constraint violations, a constrained            transaction eventually completes.    -   6. In order to determine the number of repeated iterations        required to complete a constrained transaction, the program may        employ a counter in a general register that is not subject to        the general register save mask. An example is shown below.

LH1 15, 0 Zero retry counter. Loop TBEGINC 0(0), X ‘FE00’ Preserve GRs0-13 AHI 15, 1 Increment counter . . . . . . Constrainedtransactional-execution code . . . TEND End of transaction. * R15 nowcontains count of repeated transactional attempts.

Note that both registers 14 and 15 are not restored in this example.Also note that on some models, the count in general register 15 may below if the CPU detects the abort condition following the completion ofthe TBEGINC instruction, but before the completion of the AHIinstruction.

As observed by the CPU, fetches and stores made in the transactionalexecution mode are no different than those made while not in thetransaction execution mode.

In one embodiment, the user (i.e., the one creating the transaction)selects whether or not a transaction is to be constrained. Oneembodiment of logic associated with processing constrained transactions,and, in particular, processing associated with a TBEGINC instruction,are described with reference to FIG. 11. Execution of the TBEGINCinstruction causes the CPU to enter the constrained transactionalexecution mode or remain in the nonconstrained execution mode. The CPU(i.e., the processor) executing TBEGINC performs the logic of FIG. 11.

Referring to FIG. 11, based on execution of a TBEGINC instruction, aserialization function is performed, STEP 1100. A serialization functionor operation includes completing all conceptually previous storageaccesses (and, for the z/Architecture, as an example, related referencebit and change bit settings) by the CPU, as observed by other CPUs andby the I/O subsystem, before the conceptually subsequent storageaccesses (and related reference bit and change bit settings) occur.Serialization affects the sequence of all CPU accesses to storage and tothe storage keys, except for those associated with ART table entry andDAT table entry fetching.

As observed by a CPU in the transactional execution mode, serializationoperates normally (as described above). As observed by other CPUs and bythe I/O subsystem, a serializing operation performed while a CPU is inthe transactional execution mode occurs when the CPU leaves thetransactional execution mode, either as a result of a TRANSACTION ENDinstruction that decrements the transaction nesting depth to zero(normal ending), or as a result of the transaction being aborted.

Subsequent to performing serialization, a determination is made as towhether an exception is recognized, INQUIRY 1102. If so, the exceptionis handled, STEP 1104. For instance, a special operation exception isrecognized and the operation is suppressed if the transactionalexecution control, bit 8 of control register 0, is 0. As furtherexamples, a specification exception is recognized and the operation issuppressed, if the B₁ field, bits 16-19 of the instruction, is nonzero;an execute exception is recognized and the operation is suppressed, ifthe TBEGINC is the target of an execute-type instruction; and anoperation exception is recognized and the operation is suppressed, ifthe transactional execution facility is not installed in theconfiguration. If the CPU is already in the constrained transactionexecution mode, then a transaction constrained exception programexception is recognized and the operation is suppressed. Further, if thetransaction nesting depth, when incremented by 1, would exceed a modeldependent maximum transaction nesting depth, the transaction is abortedwith abort code 13. Other or different exceptions may be recognized andhandled.

However, if there is not an exception, then a determination is made asto whether the transaction nesting depth is zero, INQUIRY 1106. If thetransaction nesting depth is zero, then the transaction diagnostic blockaddress is considered to be invalid, STEP 1108; the transaction abortPSW is set from the contents of the current PSW, except that theinstruction address of the transaction abort PSW designates the TBEGINCinstruction, rather than the next sequential instruction, STEP 1110; andthe contents of the general register pairs as designated by the generalregister save mask are saved in a model dependent location that is notdirectly accessible by the program, STEP 1112. Further, the nestingdepth is set to 1, STEP 1114. Additionally, the effective value of theallow floating point operation (F) and program interruption filteringcontrols (PIFC) are set to zero, STEP 1116. Further, the effective valueof the allow AR modification (A) control, bit 12 field of the I₂ fieldof the instruction, is determined, STEP 1118. For example, the effectiveA control is the logical AND of the A control in the TBEGINC instructionfor the current level and for any outer TBEGIN instructions.

Returning to INQUIRY 1106, if the transaction nesting depth is greaterthan zero, then the nesting depth is incremented by 1, STEP 1120.Further, the effective value of the allow floating point operation (F)is set to zero, and the effective value of the program interruptionfiltering control (PIFC) is unchanged, STEP 1122. Processing thencontinues with STEP 1118. In one embodiment, a successful initiation ofthe transaction results in condition code 0. This concludes oneembodiment of the logic associated with executing a TBEGINC instruction.

In one embodiment, the exception checking provided above can occur invarying order. One particular order for the exception checking is asfollows:

-   -   Exceptions with the same priority as the priority of        program-interruption conditions for the general case.    -   Specification exception due to the B₁ field containing a nonzero        value.    -   Abort due to exceeding transaction nesting depth.    -   Condition code 0 due to normal completion.

Additionally, the following applies in one or more embodiments:

-   -   1. Registers designated to be saved by the general register save        mask are only restored if the transaction aborts, not when the        transaction ends normally by means of TRANSACTION END. Only the        registers designated by the GRSM of the outermost TRANSACTION        BEGIN instruction are restored on abort.        -   The I₂ field should designate all register pairs that            provide input values that are changed by a constrained            transaction. Thus, if the transaction is aborted, the input            register values will be restored to their original contents            when the constrained transaction is re-executed.    -   2. On most models, improved performance may be realized, both on        TRANSACTION BEGIN and when a transaction aborts, by specifying        the minimum number of registers needed to be saved and restored        in the general register save mask.    -   3. The following illustrates the results of the TRANSACTION        BEGIN instruction (both TBEGIN and TBEGINC) based on the current        transaction nesting depth (TND) and, when the TND is nonzero,        whether the CPU is in the nonconstrained or constrained        transactional-execution mode:

Instruction TND = 0 TBEGIN Enter the nonconstrainedtransactional-execution mode TBEGINC Enter the constrainedtransactional-execution mode TND > 0 Instruction NTX Mode CTX ModeTBEGIN Continue in the nonconstrained Transaction-constrainedtransactional-execution mode exception TBEGINC Continue in thenonconstrained Transaction-constrained transactional-execution modeexception Explanation: CTX CPU is in the constrainedtransactional-execution mode NTX CPU is in the nonconstrainedtransactional-execution mode TND Transaction nesting depth at thebeginning of the instruction.

As described herein, in one aspect, a constrained transaction is assuredof completion, assuming it does not contain a condition that makes itunable to complete. To ensure it completes, the processor (e.g., CPU)executing the transaction may take certain actions. For instance, if aconstrained transaction has an abort condition, the CPU may temporarily:

-   -   (a) inhibit out-of-order execution;    -   (b) inhibit other CPUs from accessing the conflicting storage        locations;    -   (c) induce random delays in abort processing; and/or    -   (d) invoke other measures to facilitate successful completion.

To summarize, processing of a constrained transaction is, as follows:

-   -   If already in the constrained-TX mode, a transaction-constrained        exception is recognized.    -   If current TND (Transaction Nesting Depth)>0, execution proceeds        as if nonconstrained transaction        -   Effective F control set to zero        -   Effective PIFC is unchanged        -   Allows outer nonconstrained TX to call service function that            may or may not use constrained TX.    -   If current TND=0:        -   Transaction diagnostic block address is invalid            -   No instruction-specified TDB stored on abort        -   Transaction-abort PSW set to address of TBEGINC            -   Not the next sequential instruction        -   General-register pairs designated by GRSM saved in a            model-dependent location not accessible by program        -   Transaction token optionally formed (from D₂ operand). The            transaction token is an identifier of the transaction. It            may be equal to the storage operand address or another            value.    -   Effective A=TBEGINC A & any outer A    -   TND incremented        -   If TND transitions from 0 to 1, CPU enters the constrained            TX mode        -   Otherwise, CPU remains in the nonconstrained TX mode    -   Instruction completes with CC0    -   Exceptions:        -   Specification exception (PIC (Program Interruption            Code) 0006) if B₁ field is nonzero        -   Special operation exception (PIC 0013 hex) if            transaction-execution control (CR0.8) is zero        -   Transaction constraint exception (PIC 0018 hex) if issued in            constrained TX mode        -   Operation exception (PIC 0001) if the constrained            transactional execution facility is not installed        -   Execute exception (PIC 0003) if the instruction is the            target of an execute-type instruction        -   Abort code 13 if nesting depth exceeded    -   Abort conditions in constrained transaction:        -   Abort PSW points to TBEGINC instruction            -   Not the instruction following it            -   Abort condition causes entire TX to be re-driven                -   No fail path        -   CPU takes special measures to ensure successful completion            on re-drive        -   Assuming no persistent conflict, interrupt, or constrained            violation, the transaction is assured of eventual            completion.    -   Constraint violation:        -   PIC 0018 hex—indicates violation of transaction constraint        -   Or, transaction runs as if nonconstrained

As described above, in addition to constrained transaction processing,which is optional, in one embodiment, the transactional facility alsoprovides nonconstrained transaction processing. Further detailsregarding the processing of nonconstrained transactions, and, inparticular, the processing associated with a TBEGIN instruction aredescribed with reference to FIG. 12. Execution of the TBEGIN instructioncauses the CPU either to enter or to remain in the nonconstrainedtransactional execution mode. The CPU (i.e., the processor) thatexecutes TBEGIN performs the logic of FIG. 12.

Referring to FIG. 12, based on execution of the TBEGIN instruction, aserialization function (described above) is performed, STEP 1200.Subsequent to performing serialization, a determination is made as towhether an exception is recognized, INQUIRY 1202. If so, then theexception is handled, STEP 1204. For instance, a special operationexception is recognized and the operation is suppressed if thetransactional execution control, bit 8 of control register 0, is zero.Further, a specification exception is recognized and the operation issuppressed if the program interruption filtering control, bits 14-15 ofthe I₂ field of the instruction, contains the value 3; or the firstoperand address does not designate a double word boundary. An operationexception is recognized and the operation is suppressed, if thetransactional execution facility is not installed in the configuration;and an execute exception is recognized and the operation is suppressedif the TBEGIN is the target of an execute-type instruction.Additionally, if the CPU is in the constrained transactional executionmode, then a transaction constrained exception program exception isrecognized and the operation is suppressed. Further, if the transactionnesting depth, when incremented by 1, would exceed a model dependentmaximum transaction nesting depth, the transaction is aborted with abortcode 13.

Yet further, when the B₁ field of the instruction is nonzero and the CPUis not in the transactional execution mode, i.e., the transactionnesting depth is zero, then the store accessibility to the first operandis determined. If the first operand cannot be accessed for stores, thenan access exception is recognized and the operation is either nullified,suppressed, or terminated, depending on the specific access-exceptioncondition. Additionally, any PER storage alteration event for the firstoperand is recognized. When the B₁ field is nonzero and the CPU isalready in the transactional execution mode, it is unpredictable whetherstore accessibility to the first operand is determined, and PER storagealteration events are detected for the first operand. If the B₁ field iszero, then the first operand is not accessed.

In addition to the exception checking, a determination is made as towhether the CPU is in the transactional execution mode (i.e.,transaction nesting depth is zero), INQUIRY 1206. If the CPU is not inthe transactional execution mode, then the contents of selected generalregister pairs are saved, STEP 1208. In particular, the contents of thegeneral register pairs designated by the general register save mask aresaved in a model dependent location that is not directly accessible bythe program.

Further, a determination is made as to whether the B₁ field of theinstruction is zero, INQUIRY 1210. If the B₁ field is not equal to zero,the first operand address is placed in the transaction diagnostic blockaddress, STEP 1214, and the transaction diagnostic block address isvalid. Further, the transaction abort PSW is set from the contents ofthe current PSW, STEP 1216. The instruction address of the transactionabort PSW designates the next sequential instruction (that is, theinstruction following the outermost TBEGIN).

Moreover, a determination is made of the effective value of the allow ARmodification (A) control, bit 12 of the I₂ field of the instruction,STEP 1218. The effective A control is the logical AND of the A controlin the TBEGIN instruction for the current level and for all outerlevels. Additionally, an effective value of the allow floating pointoperation (F) control, bit 13 of the I₂ field of the instruction, isdetermined, STEP 1220. The effective F control is the logical AND of theF control in the TBEGIN instruction for the current level and for allouter levels. Further, an effective value of the program interruptionfiltering control (PIFC), bits 14-15 of the I₂ field of the instruction,is determined, STEP 1222. The effective PIFC value is the highest valuein the TBEGIN instruction for the current level and for all outerlevels.

Additionally, a value of one is added to the transaction nesting depth,STEP 1224, and the instruction completes with setting condition code 0,STEP 1226. If the transaction nesting depth transitions from zero toone, the CPU enters the nonconstrained transactional execution mode;otherwise, the CPU remains in the nonconstrained transactional executionmode.

Returning to INQUIRY 1210, if B₁ is equal to zero, then the transactiondiagnostic block address is invalid, STEP 1211, and processing continueswith STEP 1218. Similarly, if the CPU is in transactional executionmode, INQUIRY 1206, processing continues with STEP 1218.

Resulting Condition Code of execution of TBEGIN include, for instance:

0 Transaction initiation successful 1 — 2 — 3 —

Program Exceptions include, for instance:

-   -   Access (store, first operand)    -   Operation (transactional execution facility not installed)    -   Special operation    -   Specification    -   Transaction constraint (due to restricted instruction)

In one embodiment, the exception checking provided above can occur invarying order. One particular order to the exception checking is asfollows:

-   -   Exceptions with the same priority as the priority of program        interruption conditions for the general case.    -   Specification exception due to reserved PIFC value.    -   Specification exception due to first operand address not on a        doubleword boundary.    -   Access exception (when B₁ field is nonzero).    -   Abort due to exceeding maximum transaction nesting depth.    -   Condition code 0 due to normal completion.

Notes:

-   -   1. When the B₁ field is nonzero, the following applies:        -   An accessible transaction diagnostic block (TDB) is to be            provided when an outermost transaction is initiated—even if            the transaction never aborts.        -   Since it is unpredictable whether accessibility of the TDB            is tested for nested transactions, an accessible TDB should            be provided for any nested TBEGIN instruction.        -   The performance of any TBEGIN in which the B₁ field is            nonzero, and the performance of any abort processing that            occurs for a transaction that was initiated by an outermost            TBEGIN in which the B₁ field is nonzero, may be slower than            when the B₁ field is zero.    -   2. Registers designated to be saved by the general register save        mask are only restored, in one embodiment, if the transaction        aborts, not when the transaction ends normally by means of        TRANSACTION END. Only the registers designated by the GRSM of        the outermost TRANSACTION BEGIN instruction are restored on        abort.        -   The I₂ field should designate all register pairs that            provide input values that are changed by the transaction.            Thus, if the transaction is aborted, the input register            values will be restored to their original contents when the            abort handler is entered.    -   3. The TRANSACTION BEGIN (TBEGIN) instruction is expected to be        followed by a conditional branch instruction that will determine        whether the transaction was successfully initiated.    -   4. If a transaction is aborted due to conditions that do not        result in an interruption, the instruction designated by the        transaction abort PSW receives control (that is, the instruction        following the outermost TRANSACTION BEGIN (TBEGIN)). In addition        to the condition code set by the TRANSACTION BEGIN (TBEGIN)        instruction, condition codes 1-3 are also set when a transaction        aborts.        -   Therefore, the instruction sequence following the outermost            TRANSACTION BEGIN (TBEGIN) instruction should be able to            accommodate all four condition codes, even though the TBEGIN            instruction only sets code 0, in this example.    -   5. On most models, improved performance may be realized, both on        TRANSACTION BEGIN and when a transaction aborts, by specifying        the minimum number of registers needed to be saved and restored        in the general register save mask.    -   6. While in the nonconstrained transactional execution mode, a        program may call a service function which may alter access        registers or floating point registers (including the floating        point control register). Although such a service routine may        save the altered registers on entry and restore them at exit,        the transaction may be aborted prior to normal exit of the        routine. If the calling program makes no provision for        preserving these registers while the CPU is in the        nonconstrained transactional execution mode, it may not be able        to tolerate the service function's alteration of the registers.        -   To prevent inadvertent alteration of access registers while            in the nonconstrained transactional execution mode, the            program can set the allow AR modification control, bit 12 of            the I₂ field of the TRANSACTION BEGIN instruction, to zero.            Similarly, to prevent the inadvertent alteration of the            floating point registers, the program can set the allow            floating point operation control, bit 13 of the I₂ field of            the TBEGIN instruction, to zero.    -   7. Program exception conditions recognized during execution of        the TRANSACTION BEGIN (TBEGIN) instruction are subject to the        effective program interruption filtering control set by any        outer TBEGIN instructions. Program exception conditions        recognized during the execution of the outermost TBEGIN        instruction are not subject to filtering.    -   8. In order to update multiple storage locations in a serialized        manner, conventional code sequences may employ a lock word        (semaphore). If (a) transactional execution is used to implement        updates of multiple storage locations, (b) the program also        provides a “fall-back” path to be invoked if the transaction        aborts, and (c) the fallback path employs a lock word, then the        transactional execution path should also test for the        availability of the lock, and, if the lock is unavailable, end        the transaction by means of the TRANSACTION END instruction and        branch to the fall back path. This ensures consistent access to        the serialized resources, regardless of whether they are updated        transactionally.        -   Alternatively, the program could abort if the lock is            unavailable, however the abort processing may be            significantly slower than simply ending the transaction via            TEND.    -   9. If the effective program interruption filtering control        (PIFC) is greater than zero, the CPU filters most data exception        program interruptions. If the effective allow floating point        operation (F) control is zero, the data exception code (DXC)        will not be set in the floating point control register as a        result of an abort due to a data exception program exception        condition. In this scenario (filtering applies and the effective        F control is zero), the only location in which the DXC is        inspected is in the TBEGIN-specified TDB. If the program's abort        handler is to inspect the DXC in such a situation, general        register B₁ should be nonzero, such that a valid transaction        diagnostic block address (TDBA) is set.    -   10. If a PER storage alteration or zero address detection        condition exists for the TBEGIN-specified TDB of the outermost        TBEGIN instruction, and PER event suppression does not apply,        the PER event is recognized during the execution of the        instruction, thus causing the transaction to be aborted        immediately, regardless of whether any other abort condition        exists.

In one embodiment, the TBEGIN instruction implicitly sets thetransaction abort address to be the next sequential instructionfollowing the TBEGIN. This address is intended to be a conditionalbranch instruction which determines whether or not to branch dependingon the condition code (CC). A successful TBEGIN sets CC0, whereas anaborted transaction sets CC1, CC2, or CC3.

In one embodiment, the TBEGIN instruction provides an optional storageoperand designating the address of a transaction diagnostic block (TDB)into which information is stored if the transaction is aborted.

Further, it provides an immediate operand including the following:

-   -   A general register save mask (GRSM) indicating which pairs of        general registers are to be saved at the beginning of        transactional execution and restored if the transaction is        aborted;    -   A bit (A) to allow aborting of the transaction if the        transaction modifies access registers;    -   A bit (F) to allow aborting of the transaction if the        transaction attempts to execute floating point instructions; and    -   A program interruption filtering control (PIFC) that allows        individual transaction levels to bypass the actual presentation        of a program interruption if a transaction is aborted.

The A, F, and PIFC controls can be different at various nesting levelsand restored to the previous level when inner transaction levels areended.

Moreover, the TBEGIN (or in another embodiment, TBEGINC) is used to forma transaction token. Optionally, the token may be matched with a tokenformed by the TEND instruction. For each TBEGIN (or TBEGINC)instruction, as an example, a token is formed from the first operandaddress. This token may be formed independent of whether the baseregister is zero (unlike TDB address setting which only occurs when thebase register is nonzero). For each TRANSACTION END instruction executedwith a nonzero base register, a similar token is formed from its storageoperand. If the tokens do not match, a program exception may berecognized to alert the program of an unpaired instruction.

Token matching provides a mechanism intended to improve softwarereliability by ensuring that a TEND statement is properly paired with aTBEGIN (or TBEGINC). When a TBEGIN instruction is executed at aparticular nesting level, a token is formed from the storage operandaddress that identifies this instance of a transaction. When acorresponding TEND instruction is executed, a token is formed from thestorage operand address of the instruction, and the CPU compares thebegin token for the nesting level with the end token. If the tokens donot match, an exception condition is recognized. A model may implementtoken matching for only a certain number of nesting levels (or for nonesting levels). The token may not involve all bits of the storageoperand address, or the bits may be combined via hashing or othermethods. A token may be formed by the TBEGIN instruction even if itsstorage operand is not accessed.

To summarize, processing of a nonconstrained transaction is, as follows:

-   -   If TND=0:        -   If B₁≠0, transaction diagnostic block address set from first            operand address.        -   Transaction abort PSW set to next sequential instruction            address.        -   General register pairs designated by I₂ field are saved in            model-dependent location.            -   Not directly accessible by the program    -   Effective PIFC, A, & F controls computed        -   Effective A=TBEGIN A & any outer A        -   Effective F=TBEGIN F & any outer F        -   Effective PIFC=max(TBEGIN PIFC, any outer PIFC)    -   Transaction nesting depth (TND) incremented        -   If TND transitions from 0 to 1, CPU enters the transactional            execution mode        -   Condition code set to zero            -   When instruction following TBEGIN receives control:                -   TBEGIN success indicated by CC0                -   Aborted transaction indicated by nonzero CC    -   Exceptions:        -   Abort code 13 if nesting depth exceeded        -   Access exception (one of various PICs) if the B₁ field is            nonzero, and the storage operand cannot be accessed for a            store operation        -   Execute exception (PIC 0003) if the TBEGIN instruction is            the target of an execute-type instruction        -   Operation exception (PIC 0001) if the transactional            execution facility is not installed        -   PIC 0006 if either            -   PIFC is invalid (value of 3)            -   Second-operand address not doubleword aligned        -   PIC 0013 hex if transactional-execution control (CR0.8) is            zero        -   PIC 0018 hex if issued in constrained TX mode

As indicated above, in one embodiment, selected registers may be savedat the initiation of transactional execution, and then restored in theevent of a transaction abort. One embodiment of the logic associatedwith saving/restoring selected registers, in general, is described withreference to FIG. 13A. Further embodiments are then described withreference to FIG. 13B. As one example, a processor is executing thislogic.

Referring to FIG. 13A, a determination is made of one or more selectedregisters to be saved, STEP 1300. In one example, this determination ismade by checking the set bits of a GRSM indicated by a TRANSACTION BEGINinstruction. However, in other embodiments, other techniques may be usedto indicate selected registers to be saved. The contents of the selectedregisters are stored in a predefined memory location that isinaccessible, in one embodiment, to programs, STEP 1302.

Then, if there is an abort of a transaction associated with theregisters, INQUIRY 1304, the selected registers are restored, STEP 1306.For instance, the contents stored in the predefined memory location areloaded into the appropriate registers.

In one particular embodiment, the registers to be stored are specifiedby an outermost TRANSACTION BEGIN instruction, as described withreference to FIG. 13B. In this embodiment, a TRANSACTION BEGINinstruction is obtained (e.g., received, fetched, etc.) by a processor,STEP 1350. A determination is made as to whether it is an outermostTRANSACTION BEGIN instruction, INQUIRY 1352. If the TRANSACTION BEGINinstruction is not initiated within another transaction, and istherefore, an outermost TRANSACTION BEGIN instruction, a determinationis made using the GRSM of the instruction of the one or more registersto be saved, STEP 1354. In this example, the registers are generalregisters, and as above, they are saved by copying their contents, priorto use of the registers (e.g., right at the start of execution of theinstruction), to a predefined memory location, STEP 1356.

Returning to INQUIRY 1352, if it is not an outermost TRANSACTION BEGINinstruction, then the GRSM is ignored, in this embodiment.

Subsequent to saving any registers to be saved, processing of thetransaction proceeds, and if the transaction aborts, either because thisspecified transaction aborts or, in this embodiment, an inner nestedtransaction aborts causing this transaction to abort, INQUIRY 1358, theselected registers are restored by copying the contents from thepredefined memory location to the registers, STEP 1360. This completesprocessing.

Described above are embodiments of saving/restoring registers, such asgeneral registers, in an abort (i.e., abnormal termination). When atransaction is aborted, transactional stores made during the executionof the transaction are discarded. However, unless otherwise specified,the contents of general registers are not restored. The contents ofaccess registers and the floating-point context are also not restored,in one example, if a transaction aborts. By not saving registers unlessspecifically requested by the program, processing speed of execution ofthe outermost TRANSACTION BEGIN instruction is increased.

In one embodiment, program-specified general registers are saved duringthe execution of an outermost TRANSACTION BEGIN instruction, and thoseregisters are restored should the transaction be aborted. In a furtherembodiment, registers, other than general registers, such as floatingpoint registers and/or access registers, are saved and/or restored.Further, individual general registers and/or groupings of generalregisters may be saved/restored. Additionally, in one embodiment, ageneral purpose register that specifies a bit mask is used to specifywhich registers to set.

In yet a further embodiment, one or more of the saving/restoring isperformed in transactional environments that implement non-flattenednesting. In such an environment, when one transaction within a nest oftransactions aborts, the other transactions within the nest may notabort. In such an environment, each level of nesting has an indicationof which register(s) are to be saved and/or restored.

Provided above is an efficient means of updating multiple, discontiguousobjects in memory without classic (course-grained) serialization, suchas locking, that provides a potential for significant multiprocessorperformance improvement. That is, multiple, discontiguous objects areupdated without the enforcement of more course-grained storage-accessordering that is provided by classic techniques, such as locks andsemaphores. Speculative execution is provided without onerous recoverysetup, and constrained transactions are offered for simple,small-footprint updates.

Transactional execution can be used in a variety of scenarios,including, but not limited to, partial inlining, speculative processing,and lock elision. In partial inlining, the partial region to be includedin the executed path is wrapped in TBEGIN/TEND. TABORT can be includedtherein to roll back state on a side-exit. For speculation, such as inJava, nullchecks on de-referenced pointers can be delayed to loop edgeby using a transaction. If the pointer is null, the transaction canabort safely using TABORT, which is included within TBEGIN/TEND.

As for lock elision, one example of its use is described with referenceto FIGS. 14A-14B and the code fragment provided below.

FIG. 14A depicts a doubly linked list 1400 of a plurality of queueelements 1402 a-1402 d. A new queue element 1402 e is to be insertedinto the doubly linked list of queue elements 1400. Each queue element1402 a-1402 e includes a forward pointer 1404 a-1404 e and a backwardpointer 1406 a-1406 e. As shown in FIG. 14B, to add queue element 1402 ebetween queue elements 1402 b and 1402 c, (1) backward pointer 1406 e isset to point to queue element 1402 b, (2) forward pointer 1404 e is setto point to queue element 1402 c, (3) backward pointer 1406 c is set topoint to queue element 1402 e, and (4) forward pointer 1404 b is set topoint to queue element 1402 e.

An example code fragment corresponding to FIGS. 14A-14B is below:

NEW USING QEL, R1 CURR USING QEL, R2 LHI R15, 10 Load retry count. LOOPTBEGIN TDB, X’C000’ Begin transaction (save GRs 0-3) JNZ ABORTED NonzeroCC means aborted. LG R3, CURR.BWD Point to previous element. PREV USINGQEL, R3 Make it addressable. STG R1, PREV.FWD Update prev. forward ptr.STG R1, CURR.BWD Update curr. backward ptr. STG R2, NEW.FWD Update newforward ptr. STG R3, NEW.BWD Update new backward ptr. TEND Endtransaction. . . . ABORTED JO NO_RETRY CC3: Nonretryable abort. JCT R15,LOOP Retry transaction a few times. J NO_RETRY No joy after 10x; do itthe hard way. R1—address of the new queue element to be inserted.R2—address of the insertion point; new element is inserted before theelement pointed to by R2.

In one example, if the transaction is used for lock elision, but thefallback path uses a lock, the transaction is to at least fetch the lockword to see that it is available. The processor ensures that thetransaction aborts, if another CPU accesses the locknon-transactionally.

As used herein, storage, central storage, main storage, memory and mainmemory are used interchangeably, unless otherwise noted, implicitly byusage or explicitly. Further, while in one embodiment, a transactioneffectively delaying includes delaying committing transactional storesto main memory until completion of a selected transaction; in anotherembodiment, a transaction effectively delaying includes allowingtransactional updates to memory, but keeping the old values andrestoring memory to the old values on abort.

As will be appreciated by one skilled in the art, one or more aspectsmay be embodied as a system, method or computer program product.Accordingly, one or more aspects may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system”. Furthermore,one or more aspects may take the form of a computer program productembodied in one or more computer readable medium(s) having computerreadable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readablestorage medium. A computer readable storage medium may be, for example,but not limited to, an electronic, magnetic, optical, electromagnetic,infrared or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. More specific examples (a non-exhaustivelist) of the computer readable storage medium include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

Referring now to FIG. 15, in one example, a computer program product1500 includes, for instance, one or more non-transitory computerreadable storage media 1502 to store computer readable program codemeans or logic 1504 thereon to provide and facilitate one or moreembodiments.

Program code embodied on a computer readable medium may be transmittedusing an appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for one or moreembodiments may be written in any combination of one or more programminglanguages, including an object oriented programming language, such asJava, Smalltalk, C++ or the like, and conventional proceduralprogramming languages, such as the “C” programming language, assembleror similar programming languages. The program code may execute entirelyon the user's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

One or more embodiments are described herein with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products. It will be understood that each block of theflowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments. In this regard, each block in the flowchart or blockdiagrams may represent a module, segment, or portion of code, whichcomprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In addition to the above, one or more aspects may be provided, offered,deployed, managed, serviced, etc. by a service provider who offersmanagement of customer environments. For instance, the service providercan create, maintain, support, etc. computer code and/or a computerinfrastructure that performs one or more aspects for one or morecustomers. In return, the service provider may receive payment from thecustomer under a subscription and/or fee agreement, as examples.Additionally or alternatively, the service provider may receive paymentfrom the sale of advertising content to one or more third parties.

In one aspect, an application may be deployed for performing one or moreembodiments. As one example, the deploying of an application comprisesproviding computer infrastructure operable to perform one or moreembodiments.

As a further aspect, a computing infrastructure may be deployedcomprising integrating computer readable code into a computing system,in which the code in combination with the computing system is capable ofperforming one or more embodiments.

As yet a further aspect, a process for integrating computinginfrastructure comprising integrating computer readable code into acomputer system may be provided. The computer system comprises acomputer readable medium, in which the computer medium comprises one ormore embodiments. The code in combination with the computer system iscapable of performing one or more embodiments.

Although various embodiments are described above, these are onlyexamples. For example, computing environments of other architectures canbe used. Further, different instructions, instruction formats,instruction fields and/or instruction values may be used. Moreover,different, other, and/or additional restrictions/constraints may beprovided/used. Yet further, registers other than general registers maybe saved/restored. Further, the saving/restoring can be performed bydifferent processors, or by one or more processors. Many variations arepossible.

Further, other types of computing environments can be used. As anexample, a data processing system suitable for storing and/or executingprogram code is usable that includes at least two processors coupleddirectly or indirectly to memory elements through a system bus. Thememory elements include, for instance, local memory employed duringactual execution of the program code, bulk storage, and cache memorywhich provide temporary storage of at least some program code in orderto reduce the number of times code must be retrieved from bulk storageduring execution.

Input/Output or I/O devices (including, but not limited to, keyboards,displays, pointing devices, DASD, tape, CDs, DVDs, thumb drives andother memory media, etc.) can be coupled to the system either directlyor through intervening I/O controllers. Network adapters may also becoupled to the system to enable the data processing system to becomecoupled to other data processing systems or remote printers or storagedevices through intervening private or public networks. Modems, cablemodems, and Ethernet cards are just a few of the available types ofnetwork adapters.

Referring to FIG. 16, representative components of a Host Computersystem 5000 to implement one or more embodiments. The representativehost computer 5000 comprises one or more CPUs 5001 in communication withcomputer memory (i.e., central storage) 5002, as well as I/O interfacesto storage media devices 5011 and networks 5010 for communicating withother computers or SANs and the like. The CPU 5001 is compliant with anarchitecture having an architected instruction set and architectedfunctionality. The CPU 5001 may have access register translation (ART)5012, which includes an ART lookaside buffer (ALB) 5013, for selectingan address space to be used by dynamic address translation (DAT) 5003for transforming program addresses (virtual addresses) into realaddresses of memory. A DAT typically includes a translation lookasidebuffer (TLB) 5007 for caching translations so that later accesses to theblock of computer memory 5002 do not require the delay of addresstranslation. Typically, a cache 5009 is employed between computer memory5002 and the processor 5001. The cache 5009 may be hierarchical having alarge cache available to more than one CPU and smaller, faster (lowerlevel) caches between the large cache and each CPU. In someimplementations, the lower level caches are split to provide separatelow level caches for instruction fetching and data accesses. In oneembodiment, for the TX facility, a transaction diagnostic block (TDB)5100 and one or more buffers 5101 may be stored in one or more of cache5009 and memory 5002. In one example, in TX mode, data is initiallystored in a TX buffer, and when TX mode ends (e.g., outermost TEND), thedata in the buffer is stored (committed) to memory, or if there is anabort, the data in the buffer is discarded.

In one embodiment, an instruction is fetched from memory 5002 by aninstruction fetch unit 5004 via a cache 5009. The instruction is decodedin an instruction decode unit 5006 and dispatched (with otherinstructions in some embodiments) to instruction execution unit or units5008. Typically several execution units 5008 are employed, for examplean arithmetic execution unit, a floating point execution unit and abranch instruction execution unit. Further, in one embodiment of the TXfacility, various TX controls 5110 may be employed. The instruction isexecuted by the execution unit, accessing operands from instructionspecified registers or memory as needed. If an operand is to be accessed(loaded or stored) from memory 5002, a load/store unit 5005 typicallyhandles the access under control of the instruction being executed.Instructions may be executed in hardware circuits or in internalmicrocode (firmware) or by a combination of both.

In accordance with an aspect of the TX facility, processor 5001 alsoincludes a PSW 5102 (e.g., TX and/or abort PSW), a nesting depth 5104, aTDBA 5106, and one or more control registers 5108.

As noted, a computer system includes information in local (or main)storage, as well as addressing, protection, and reference and changerecording. Some aspects of addressing include the format of addresses,the concept of address spaces, the various types of addresses, and themanner in which one type of address is translated to another type ofaddress. Some of main storage includes permanently assigned storagelocations. Main storage provides the system with directly addressablefast-access storage of data. Both data and programs are to be loadedinto main storage (from input devices) before they can be processed.

Main storage may include one or more smaller, faster-access bufferstorages, sometimes called caches. A cache is typically physicallyassociated with a CPU or an I/O processor. The effects, except onperformance, of the physical construction and use of distinct storagemedia are generally not observable by the program.

Separate caches may be maintained for instructions and for dataoperands. Information within a cache is maintained in contiguous byteson an integral boundary called a cache block or cache line (or line, forshort). A model may provide an EXTRACT CACHE ATTRIBUTE instruction whichreturns the size of a cache line in bytes. A model may also providePREFETCH DATA and PREFETCH DATA RELATIVE LONG instructions which effectsthe prefetching of storage into the data or instruction cache or thereleasing of data from the cache.

Storage is viewed as a long horizontal string of bits. For mostoperations, accesses to storage proceed in a left-to-right sequence. Thestring of bits is subdivided into units of eight bits. An eight-bit unitis called a byte, which is the basic building block of all informationformats. Each byte location in storage is identified by a uniquenonnegative integer, which is the address of that byte location or,simply, the byte address. Adjacent byte locations have consecutiveaddresses, starting with 0 on the left and proceeding in a left-to-rightsequence. Addresses are unsigned binary integers and are 24, 31, or 64bits.

Information is transmitted between storage and a CPU or a channelsubsystem one byte, or a group of bytes, at a time. Unless otherwisespecified, in, for instance, the z/Architecture, a group of bytes instorage is addressed by the leftmost byte of the group. The number ofbytes in the group is either implied or explicitly specified by theoperation to be performed. When used in a CPU operation, a group ofbytes is called a field. Within each group of bytes, in, for instance,the z/Architecture, bits are numbered in a left-to-right sequence. Inthe z/Architecture, the leftmost bits are sometimes referred to as the“high-order” bits and the rightmost bits as the “low-order” bits. Bitnumbers are not storage addresses, however. Only bytes can be addressed.To operate on individual bits of a byte in storage, the entire byte isaccessed. The bits in a byte are numbered 0 through 7, from left toright (in, e.g., the z/Architecture). The bits in an address may benumbered 8-31 or 40-63 for 24-bit addresses, or 1-31 or 33-63 for 31-bitaddresses; they are numbered 0-63 for 64-bit addresses. In one example,bits 8-31 and 1-31 apply to addresses that are in a location (e.g.,register) that is 32 bits wide, whereas bits 40-63 and 33-63 apply toaddresses that are in a 64-bit wide location. Within any otherfixed-length format of multiple bytes, the bits making up the format areconsecutively numbered starting from 0. For purposes of error detection,and in preferably for correction, one or more check bits may betransmitted with each byte or with a group of bytes. Such check bits aregenerated automatically by the machine and cannot be directly controlledby the program. Storage capacities are expressed in number of bytes.When the length of a storage-operand field is implied by the operationcode of an instruction, the field is said to have a fixed length, whichcan be one, two, four, eight, or sixteen bytes. Larger fields may beimplied for some instructions. When the length of a storage-operandfield is not implied but is stated explicitly, the field is said to havea variable length. Variable-length operands can vary in length byincrements of one byte (or with some instructions, in multiples of twobytes or other multiples). When information is placed in storage, thecontents of only those byte locations are replaced that are included inthe designated field, even though the width of the physical path tostorage may be greater than the length of the field being stored.

Certain units of information are to be on an integral boundary instorage. A boundary is called integral for a unit of information whenits storage address is a multiple of the length of the unit in bytes.Special names are given to fields of 2, 4, 8, 16, and 32 bytes on anintegral boundary. A halfword is a group of two consecutive bytes on atwo-byte boundary and is the basic building block of instructions. Aword is a group of four consecutive bytes on a four-byte boundary. Adoubleword is a group of eight consecutive bytes on an eight-byteboundary. A quadword is a group of 16 consecutive bytes on a 16-byteboundary. An octoword is a group of 32 consecutive bytes on a 32-byteboundary. When storage addresses designate halfwords, words,doublewords, quadwords, and octowords, the binary representation of theaddress contains one, two, three, four, or five rightmost zero bits,respectively. Instructions are to be on two-byte integral boundaries.The storage operands of most instructions do not have boundary-alignmentrequirements.

On devices that implement separate caches for instructions and dataoperands, a significant delay may be experienced if the program storesinto a cache line from which instructions are subsequently fetched,regardless of whether the store alters the instructions that aresubsequently fetched.

In one example, embodiments may be practiced by software (sometimesreferred to licensed internal code, firmware, micro-code, milli-code,pico-code and the like, any of which would be consistent with one ormore embodiments). Referring to FIG. 16, software program code whichembodies one or more aspects may be accessed by processor 5001 of thehost system 5000 from long-term storage media devices 5011, such as aCD-ROM drive, tape drive or hard drive. The software program code may beembodied on any of a variety of known media for use with a dataprocessing system, such as a diskette, hard drive, or CD-ROM. The codemay be distributed on such media, or may be distributed to users fromcomputer memory 5002 or storage of one computer system over a network5010 to other computer systems for use by users of such other systems.

The software program code includes an operating system which controlsthe function and interaction of the various computer components and oneor more application programs. Program code is normally paged fromstorage media device 5011 to the relatively higher-speed computerstorage 5002 where it is available for processing by processor 5001. Thetechniques and methods for embodying software program code in memory, onphysical media, and/or distributing software code via networks are wellknown and will not be further discussed herein. Program code, whencreated and stored on a tangible medium (including but not limited toelectronic memory modules (RAM), flash memory, Compact Discs (CDs),DVDs, Magnetic Tape and the like is often referred to as a “computerprogram product”. The computer program product medium is typicallyreadable by a processing circuit preferably in a computer system forexecution by the processing circuit.

FIG. 17 illustrates a representative workstation or server hardwaresystem in which one or more embodiments may be practiced. The system5020 of FIG. 17 comprises a representative base computer system 5021,such as a personal computer, a workstation or a server, includingoptional peripheral devices. The base computer system 5021 includes oneor more processors 5026 and a bus employed to connect and enablecommunication between the processor(s) 5026 and the other components ofthe system 5021 in accordance with known techniques. The bus connectsthe processor 5026 to memory 5025 and long-term storage 5027 which caninclude a hard drive (including any of magnetic media, CD, DVD and FlashMemory for example) or a tape drive for example. The system 5021 mightalso include a user interface adapter, which connects the microprocessor5026 via the bus to one or more interface devices, such as a keyboard5024, a mouse 5023, a printer/scanner 5030 and/or other interfacedevices, which can be any user interface device, such as a touchsensitive screen, digitized entry pad, etc. The bus also connects adisplay device 5022, such as an LCD screen or monitor, to themicroprocessor 5026 via a display adapter.

The system 5021 may communicate with other computers or networks ofcomputers by way of a network adapter capable of communicating 5028 witha network 5029. Example network adapters are communications channels,token ring, Ethernet or modems. Alternatively, the system 5021 maycommunicate using a wireless interface, such as a CDPD (cellular digitalpacket data) card. The system 5021 may be associated with such othercomputers in a Local Area Network (LAN) or a Wide Area Network (WAN), orthe system 5021 can be a client in a client/server arrangement withanother computer, etc. All of these configurations, as well as theappropriate communications hardware and software, are known in the art.

FIG. 18 illustrates a data processing network 5040 in which one or moreembodiments may be practiced. The data processing network 5040 mayinclude a plurality of individual networks, such as a wireless networkand a wired network, each of which may include a plurality of individualworkstations 5041, 5042, 5043, 5044. Additionally, as those skilled inthe art will appreciate, one or more LANs may be included, where a LANmay comprise a plurality of intelligent workstations coupled to a hostprocessor.

Still referring to FIG. 18, the networks may also include mainframecomputers or servers, such as a gateway computer (client server 5046) orapplication server (remote server 5048 which may access a datarepository and may also be accessed directly from a workstation 5045). Agateway computer 5046 serves as a point of entry into each individualnetwork. A gateway is needed when connecting one networking protocol toanother. The gateway 5046 may be preferably coupled to another network(the Internet 5047 for example) by means of a communications link. Thegateway 5046 may also be directly coupled to one or more workstations5041, 5042, 5043, 5044 using a communications link. The gateway computermay be implemented utilizing an IBM eServer System z server availablefrom International Business Machines Corporation.

Referring concurrently to FIG. 17 and FIG. 18, software programming code5031 which may embody one or more embodiments may be accessed by theprocessor 5026 of the system 5020 from long-term storage media 5027,such as a CD-ROM drive or hard drive. The software programming code maybe embodied on any of a variety of known media for use with a dataprocessing system, such as a diskette, hard drive, or CD-ROM. The codemay be distributed on such media, or may be distributed to users 5050,5051 from the memory or storage of one computer system over a network toother computer systems for use by users of such other systems.

Alternatively, the programming code may be embodied in the memory 5025,and accessed by the processor 5026 using the processor bus. Suchprogramming code includes an operating system which controls thefunction and interaction of the various computer components and one ormore application programs 5032. Program code is normally paged fromstorage media 5027 to high-speed memory 5025 where it is available forprocessing by the processor 5026. The techniques and methods forembodying software programming code in memory, on physical media, and/ordistributing software code via networks are well known and will not befurther discussed herein. Program code, when created and stored on atangible medium (including but not limited to electronic memory modules(RAM), flash memory, Compact Discs (CDs), DVDs, Magnetic Tape and thelike is often referred to as a “computer program product”. The computerprogram product medium is typically readable by a processing circuitpreferably in a computer system for execution by the processing circuit.

The cache that is most readily available to the processor (normallyfaster and smaller than other caches of the processor) is the lowest (L1or level one) cache and main store (main memory) is the highest levelcache (L3 if there are 3 levels). The lowest level cache is oftendivided into an instruction cache (I-Cache) holding machine instructionsto be executed and a data cache (D-Cache) holding data operands.

Referring to FIG. 19, an exemplary processor embodiment is depicted forprocessor 5026. Typically one or more levels of cache 5053 are employedto buffer memory blocks in order to improve processor performance. Thecache 5053 is a high speed buffer holding cache lines of memory datathat are likely to be used. Typical cache lines are 64, 128 or 256 bytesof memory data. Separate caches are often employed for cachinginstructions than for caching data. Cache coherence (synchronization ofcopies of lines in memory and the caches) is often provided by various“snoop” algorithms well known in the art. Main memory storage 5025 of aprocessor system is often referred to as a cache. In a processor systemhaving 4 levels of cache 5053, main storage 5025 is sometimes referredto as the level 5 (L5) cache since it is typically faster and only holdsa portion of the non-volatile storage (DASD, tape etc) that is availableto a computer system. Main storage 5025 “caches” pages of data paged inand out of the main storage 5025 by the operating system.

A program counter (instruction counter) 5061 keeps track of the addressof the current instruction to be executed. A program counter in az/Architecture processor is 64 bits and can be truncated to 31 or 24bits to support prior addressing limits. A program counter is typicallyembodied in a PSW (program status word) of a computer such that itpersists during context switching. Thus, a program in progress, having aprogram counter value, may be interrupted by, for example, the operatingsystem (context switch from the program environment to the operatingsystem environment). The PSW of the program maintains the programcounter value while the program is not active, and the program counter(in the PSW) of the operating system is used while the operating systemis executing. Typically, the program counter is incremented by an amountequal to the number of bytes of the current instruction. RISC (ReducedInstruction Set Computing) instructions are typically fixed length whileCISC (Complex Instruction Set Computing) instructions are typicallyvariable length. Instructions of the IBM z/Architecture are CISCinstructions having a length of 2, 4 or 6 bytes. The Program counter5061 is modified by either a context switch operation or a branch takenoperation of a branch instruction for example. In a context switchoperation, the current program counter value is saved in the programstatus word along with other state information about the program beingexecuted (such as condition codes), and a new program counter value isloaded pointing to an instruction of a new program module to beexecuted. A branch taken operation is performed in order to permit theprogram to make decisions or loop within the program by loading theresult of the branch instruction into the program counter 5061.

Typically an instruction fetch unit 5055 is employed to fetchinstructions on behalf of the processor 5026. The fetch unit eitherfetches “next sequential instructions”, target instructions of branchtaken instructions, or first instructions of a program following acontext switch. Modern Instruction fetch units often employ prefetchtechniques to speculatively prefetch instructions based on thelikelihood that the prefetched instructions might be used. For example,a fetch unit may fetch 16 bytes of instruction that includes the nextsequential instruction and additional bytes of further sequentialinstructions.

The fetched instructions are then executed by the processor 5026. In anembodiment, the fetched instruction(s) are passed to a dispatch unit5056 of the fetch unit. The dispatch unit decodes the instruction(s) andforwards information about the decoded instruction(s) to appropriateunits 5057, 5058, 5060. An execution unit 5057 will typically receiveinformation about decoded arithmetic instructions from the instructionfetch unit 5055 and will perform arithmetic operations on operandsaccording to the opcode of the instruction. Operands are provided to theexecution unit 5057 preferably either from memory 5025, architectedregisters 5059 or from an immediate field of the instruction beingexecuted. Results of the execution, when stored, are stored either inmemory 5025, registers 5059 or in other machine hardware (such ascontrol registers, PSW registers and the like).

Virtual addresses are transformed into real addresses using dynamicaddress translation 5062 and, optionally, using access registertranslation 5063.

A processor 5026 typically has one or more units 5057, 5058, 5060 forexecuting the function of the instruction. Referring to FIG. 20A, anexecution unit 5057 may communicate 5071 with architected generalregisters 5059, a decode/dispatch unit 5056, a load store unit 5060, andother 5065 processor units by way of interfacing logic 5071. Anexecution unit 5057 may employ several register circuits 5067, 5068,5069 to hold information that the arithmetic logic unit (ALU) 5066 willoperate on. The ALU performs arithmetic operations such as add,subtract, multiply and divide as well as logical function such as and,or and exclusive-or (XOR), rotate and shift. Preferably the ALU supportsspecialized operations that are design dependent. Other circuits mayprovide other architected facilities 5072 including condition codes andrecovery support logic for example. Typically the result of an ALUoperation is held in an output register circuit 5070 which can forwardthe result to a variety of other processing functions. There are manyarrangements of processor units, the present description is onlyintended to provide a representative understanding of one embodiment.

An ADD instruction for example would be executed in an execution unit5057 having arithmetic and logical functionality while a floating pointinstruction for example would be executed in a floating point executionhaving specialized floating point capability. Preferably, an executionunit operates on operands identified by an instruction by performing anopcode defined function on the operands. For example, an ADD instructionmay be executed by an execution unit 5057 on operands found in tworegisters 5059 identified by register fields of the instruction.

The execution unit 5057 performs the arithmetic addition on two operandsand stores the result in a third operand where the third operand may bea third register or one of the two source registers. The execution unitpreferably utilizes an Arithmetic Logic Unit (ALU) 5066 that is capableof performing a variety of logical functions such as Shift, Rotate, And,Or and XOR as well as a variety of algebraic functions including any ofadd, subtract, multiply, divide. Some ALUs 5066 are designed for scalaroperations and some for floating point. Data may be Big Endian (wherethe least significant byte is at the highest byte address) or LittleEndian (where the least significant byte is at the lowest byte address)depending on architecture. The IBM z/Architecture is Big Endian. Signedfields may be sign and magnitude, 1's complement or 2's complementdepending on architecture. A 2's complement number is advantageous inthat the ALU does not need to design a subtract capability since eithera negative value or a positive value in 2's complement requires only anaddition within the ALU. Numbers are commonly described in shorthand,where a 12 bit field defines an address of a 4,096 byte block and iscommonly described as a 4 Kbyte (Kilo-byte) block, for example.

Referring to FIG. 20B, branch instruction information for executing abranch instruction is typically sent to a branch unit 5058 which oftenemploys a branch prediction algorithm such as a branch history table5082 to predict the outcome of the branch before other conditionaloperations are complete. The target of the current branch instructionwill be fetched and speculatively executed before the conditionaloperations are complete. When the conditional operations are completedthe speculatively executed branch instructions are either completed ordiscarded based on the conditions of the conditional operation and thespeculated outcome. A typical branch instruction may test conditioncodes and branch to a target address if the condition codes meet thebranch requirement of the branch instruction, a target address may becalculated based on several numbers including ones found in registerfields or an immediate field of the instruction for example. The branchunit 5058 may employ an ALU 5074 having a plurality of input registercircuits 5075, 5076, 5077 and an output register circuit 5080. Thebranch unit 5058 may communicate 5081 with general registers 5059,decode dispatch unit 5056 or other circuits 5073, for example.

The execution of a group of instructions can be interrupted for avariety of reasons including a context switch initiated by an operatingsystem, a program exception or error causing a context switch, an I/Ointerruption signal causing a context switch or multi-threading activityof a plurality of programs (in a multi-threaded environment), forexample. Preferably a context switch action saves state informationabout a currently executing program and then loads state informationabout another program being invoked. State information may be saved inhardware registers or in memory for example. State informationpreferably comprises a program counter value pointing to a nextinstruction to be executed, condition codes, memory translationinformation and architected register content. A context switch activitycan be exercised by hardware circuits, application programs, operatingsystem programs or firmware code (microcode, pico-code or licensedinternal code (LIC)) alone or in combination.

A processor accesses operands according to instruction defined methods.The instruction may provide an immediate operand using the value of aportion of the instruction, may provide one or more register fieldsexplicitly pointing to either general purpose registers or specialpurpose registers (floating point registers for example). Theinstruction may utilize implied registers identified by an opcode fieldas operands. The instruction may utilize memory locations for operands.A memory location of an operand may be provided by a register, animmediate field, or a combination of registers and immediate field asexemplified by the z/Architecture long displacement facility wherein theinstruction defines a base register, an index register and an immediatefield (displacement field) that are added together to provide theaddress of the operand in memory for example. Location herein typicallyimplies a location in main memory (main storage) unless otherwiseindicated.

Referring to FIG. 20C, a processor accesses storage using a load/storeunit 5060. The load/store unit 5060 may perform a load operation byobtaining the address of the target operand in memory 5053 and loadingthe operand in a register 5059 or another memory 5053 location, or mayperform a store operation by obtaining the address of the target operandin memory 5053 and storing data obtained from a register 5059 or anothermemory 5053 location in the target operand location in memory 5053. Theload/store unit 5060 may be speculative and may access memory in asequence that is out-of-order relative to instruction sequence, howeverthe load/store unit 5060 is to maintain the appearance to programs thatinstructions were executed in order. A load/store unit 5060 maycommunicate 5084 with general registers 5059, decode/dispatch unit 5056,cache/memory interface 5053 or other elements 5083 and comprises variousregister circuits 5086, 5087, 5088 and 5089, ALUs 5085 and control logic5090 to calculate storage addresses and to provide pipeline sequencingto keep operations in-order. Some operations may be out of order but theload/store unit provides functionality to make the out of orderoperations to appear to the program as having been performed in order,as is well known in the art.

Preferably addresses that an application program “sees” are oftenreferred to as virtual addresses. Virtual addresses are sometimesreferred to as “logical addresses” and “effective addresses”. Thesevirtual addresses are virtual in that they are redirected to physicalmemory location by one of a variety of dynamic address translation (DAT)technologies including, but not limited to, simply prefixing a virtualaddress with an offset value, translating the virtual address via one ormore translation tables, the translation tables preferably comprising atleast a segment table and a page table alone or in combination,preferably, the segment table having an entry pointing to the pagetable. In the z/Architecture, a hierarchy of translation is providedincluding a region first table, a region second table, a region thirdtable, a segment table and an optional page table. The performance ofthe address translation is often improved by utilizing a translationlookaside buffer (TLB) which comprises entries mapping a virtual addressto an associated physical memory location. The entries are created whenthe DAT translates a virtual address using the translation tables.Subsequent use of the virtual address can then utilize the entry of thefast TLB rather than the slow sequential translation table accesses. TLBcontent may be managed by a variety of replacement algorithms includingLRU (Least Recently used).

In the case where the processor is a processor of a multi-processorsystem, each processor has responsibility to keep shared resources, suchas I/O, caches, TLBs and memory, interlocked for coherency. Typically,“snoop” technologies will be utilized in maintaining cache coherency. Ina snoop environment, each cache line may be marked as being in any oneof a shared state, an exclusive state, a changed state, an invalid stateand the like in order to facilitate sharing.

I/O units 5054 (FIG. 19) provide the processor with means for attachingto peripheral devices including tape, disc, printers, displays, andnetworks for example. I/O units are often presented to the computerprogram by software drivers. In mainframes, such as the System z fromIBM®, channel adapters and open system adapters are I/O units of themainframe that provide the communications between the operating systemand peripheral devices.

Further, other types of computing environments can benefit from one ormore aspects. As an example, an environment may include an emulator(e.g., software or other emulation mechanisms), in which a particulararchitecture (including, for instance, instruction execution,architected functions, such as address translation, and architectedregisters) or a subset thereof is emulated (e.g., on a native computersystem having a processor and memory). In such an environment, one ormore emulation functions of the emulator can implement one or moreembodiments, even though a computer executing the emulator may have adifferent architecture than the capabilities being emulated. As oneexample, in emulation mode, the specific instruction or operation beingemulated is decoded, and an appropriate emulation function is built toimplement the individual instruction or operation.

In an emulation environment, a host computer includes, for instance, amemory to store instructions and data; an instruction fetch unit tofetch instructions from memory and to optionally, provide localbuffering for the fetched instruction; an instruction decode unit toreceive the fetched instructions and to determine the type ofinstructions that have been fetched; and an instruction execution unitto execute the instructions. Execution may include loading data into aregister from memory; storing data back to memory from a register; orperforming some type of arithmetic or logical operation, as determinedby the decode unit. In one example, each unit is implemented insoftware. For instance, the operations being performed by the units areimplemented as one or more subroutines within emulator software.

More particularly, in a mainframe, architected machine instructions areused by programmers, usually today “C” programmers, often by way of acompiler application. These instructions stored in the storage mediummay be executed natively in a z/Architecture IBM® Server, oralternatively in machines executing other architectures. They can beemulated in the existing and in future IBM® mainframe servers and onother machines of IBM® (e.g., Power Systems servers and System xServers). They can be executed in machines running Linux on a widevariety of machines using hardware manufactured by IBM®, Intel®, AMD,and others. Besides execution on that hardware under a z/Architecture,Linux can be used as well as machines which use emulation by Hercules,UMX, or FSI (Fundamental Software, Inc), where generally execution is inan emulation mode. In emulation mode, emulation software is executed bya native processor to emulate the architecture of an emulated processor.

The native processor typically executes emulation software comprisingeither firmware or a native operating system to perform emulation of theemulated processor. The emulation software is responsible for fetchingand executing instructions of the emulated processor architecture. Theemulation software maintains an emulated program counter to keep trackof instruction boundaries. The emulation software may fetch one or moreemulated machine instructions at a time and convert the one or moreemulated machine instructions to a corresponding group of native machineinstructions for execution by the native processor. These convertedinstructions may be cached such that a faster conversion can beaccomplished. Notwithstanding, the emulation software is to maintain thearchitecture rules of the emulated processor architecture so as toassure operating systems and applications written for the emulatedprocessor operate correctly. Furthermore, the emulation software is toprovide resources identified by the emulated processor architectureincluding, but not limited to, control registers, general purposeregisters, floating point registers, dynamic address translationfunction including segment tables and page tables for example, interruptmechanisms, context switch mechanisms, Time of Day (TOD) clocks andarchitected interfaces to I/O subsystems such that an operating systemor an application program designed to run on the emulated processor, canbe run on the native processor having the emulation software.

A specific instruction being emulated is decoded, and a subroutine iscalled to perform the function of the individual instruction. Anemulation software function emulating a function of an emulatedprocessor is implemented, for example, in a “C” subroutine or driver, orsome other method of providing a driver for the specific hardware aswill be within the skill of those in the art after understanding thedescription of the preferred embodiment. Various software and hardwareemulation patents including, but not limited to U.S. Pat. No. 5,551,013,entitled “Multiprocessor for Hardware Emulation”, by Beausoleil et al.;and U.S. Pat. No. 6,009,261, entitled “Preprocessing of Stored TargetRoutines for Emulating Incompatible Instructions on a Target Processor”,by Scalzi et al; and U.S. Pat. No. 5,574,873, entitled “Decoding GuestInstruction to Directly Access Emulation Routines that Emulate the GuestInstructions”, by Davidian et al; and U.S. Pat. No. 6,308,255, entitled“Symmetrical Multiprocessing Bus and Chipset Used for CoprocessorSupport Allowing Non-Native Code to Run in a System”, by Gorishek et al;and U.S. Pat. No. 6,463,582, entitled “Dynamic Optimizing Object CodeTranslator for Architecture Emulation and Dynamic Optimizing Object CodeTranslation Method”, by Lethin et al; and U.S. Pat. No. 5,790,825,entitled “Method for Emulating Guest Instructions on a Host ComputerThrough Dynamic Recompilation of Host Instructions”, by Eric Traut, eachof which is hereby incorporated herein by reference in its entirety; andmany others, illustrate a variety of known ways to achieve emulation ofan instruction format architected for a different machine for a targetmachine available to those skilled in the art.

In FIG. 21, an example of an emulated host computer system 5092 isprovided that emulates a host computer system 5000′ of a hostarchitecture. In the emulated host computer system 5092, the hostprocessor (CPU) 5091 is an emulated host processor (or virtual hostprocessor) and comprises an emulation processor 5093 having a differentnative instruction set architecture than that of the processor 5091 ofthe host computer 5000′. The emulated host computer system 5092 hasmemory 5094 accessible to the emulation processor 5093. In the exampleembodiment, the memory 5094 is partitioned into a host computer memory5096 portion and an emulation routines 5097 portion. The host computermemory 5096 is available to programs of the emulated host computer 5092according to host computer architecture. The emulation processor 5093executes native instructions of an architected instruction set of anarchitecture other than that of the emulated processor 5091, the nativeinstructions obtained from emulation routines memory 5097, and mayaccess a host instruction for execution from a program in host computermemory 5096 by employing one or more instruction(s) obtained in asequence & access/decode routine which may decode the hostinstruction(s) accessed to determine a native instruction executionroutine for emulating the function of the host instruction accessed.Other facilities that are defined for the host computer system 5000′architecture may be emulated by architected facilities routines,including such facilities as general purpose registers, controlregisters, dynamic address translation and I/O subsystem support andprocessor cache, for example. The emulation routines may also takeadvantage of functions available in the emulation processor 5093 (suchas general registers and dynamic translation of virtual addresses) toimprove performance of the emulation routines. Special hardware andoff-load engines may also be provided to assist the processor 5093 inemulating the function of the host computer 5000′.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises” and/or “comprising”,when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below, if any, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of one or more embodiments has been presentedfor purposes of illustration and description, but is not intended to beexhaustive or limited to the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain variousaspects and the practical application, and to enable others of ordinaryskill in the art to understand various embodiments with variousmodifications as are suited to the particular use contemplated.

What is claimed is:
 1. A computer system for facilitating processing oftransactions within a computing environment, said computer systemcomprising: a memory; and a processor in communications with the memory,wherein the computer system is configured to perform a method, saidmethod comprising: obtaining a transaction begin instruction, thetransaction begin instruction comprising an opcode that specifies atransaction begin instruction of a particular type, and a field thatspecifies a general register save mask to explicitly specify a set ofindividually selected registers to be saved based on executing thetransaction begin instruction, the transaction begin instruction toinitiate a transaction, and the general register save mask explicitlyspecifying the set of individually selected registers to be saved at astart of the transaction, wherein the general register save maskcomprises a plurality of bits, and each bit of the plurality of bitsrepresents an even-odd pair of registers, the transaction effectivelydelaying committing transactional stores to main memory until completionof a selected transaction; and executing the transaction begininstruction, the executing the transaction begin instruction comprisingsaving contents of the set of individually selected registers explicitlyspecified in the general register save mask of the transaction begininstruction, and initiating execution of the transaction.
 2. Thecomputer system of claim 1, wherein the method further comprises:determining that the transaction has aborted; and based on determiningthe transaction has aborted and that one or more selected registers arespecified by the general register save mask, restoring the one or moreselected registers.
 3. The computer system of claim 2, wherein thesaving comprises copying contents of the one or more selected registersto a designated memory location, and wherein the restoring comprisescopying the contents from the designated memory location to the one ormore selected registers.
 4. The computer system of claim 1, wherein theinstruction is an outermost nested transaction begin instruction of anested transaction, and wherein the method further comprises aborting atransaction of the nested transaction, and based on the aborting and oneor more selected registers being specified by the general register savemask, restoring the one or more selected registers to the contents savedbased on execution of the outermost nested transaction begininstruction.
 5. The computer system of claim 1, wherein the instructioncomprises a transaction begin instruction or a transaction beginconstrained instruction.
 6. The computer system of claim 1, wherein theset of individually selected registers comprise at least one of: one ormore general registers, one or more floating point registers, or one ormore access registers.
 7. A computer program product for facilitatingprocessing of transactions within a computing environment, said computerprogram product comprising: a non-transitory computer readable storagemedium readable by a processing circuit and storing instructions forexecution by the processing circuit for performing a method comprising:obtaining a transaction begin instruction, the transaction begininstruction comprising an opcode that specifies a transaction begininstruction of a particular type, and a field that specifies a generalregister save mask to explicitly specify a set of individually selectedregisters to be saved based on executing the transaction begininstruction, the transaction begin instruction to initiate atransaction, and the general register save mask explicitly specifyingthe set of individually selected registers to be saved at a start of thetransaction, wherein the general register save mask comprises aplurality of bits, and each bit of the plurality of bits represents aneven-odd pair of registers, the transaction effectively delayingcommitting transactional stores to main memory until completion of aselected transaction; and executing the transaction begin instruction,the executing the transaction begin instruction comprising savingcontents of the set of individually selected registers explicitlyspecified in the general register save mask of the transaction begininstruction, and initiating execution of the transaction.
 8. Thecomputer program product of claim 7, wherein the method furthercomprises: determining that the transaction has aborted; and based ondetermining the transaction has aborted and that one or more selectedregisters are specified by the general register save mask, restoring theone or more selected registers.
 9. The computer program product of claim8, wherein the saving comprises copying contents of the one or moreselected registers to a designated memory location, and wherein therestoring comprises copying the contents from the designated memorylocation to the one or more selected registers.
 10. The computer programproduct of claim 7, wherein the instruction is an outermost nestedtransaction begin instruction of a nested transaction, and wherein themethod further comprises aborting a transaction of the nestedtransaction, and based on the aborting and one or more selectedregisters being specified by the general register save mask, restoringthe one or more selected registers to the contents saved based onexecution of the outermost nested transaction begin instruction.
 11. Thecomputer program product of claim 7, wherein the instruction comprises atransaction begin instruction or a transaction begin constrainedinstruction.
 12. The computer program product of claim 7, wherein theset of individually selected registers comprise at least one of: one ormore general registers, one or more floating point registers, or one ormore access registers.